Thanks to Kriegor for helping out with the quick guide. Go check out vids he did covering the history of the genre on TEU : [part 1] [part 2]
When learning a new beat em up, one of the most important, basic things is figuring out the function of your moves so you can start playing deliberately. There are some common archetypes that you can use as a learning shortcut. These mostly apply to belt scrolling beat 'em ups rather than single plane ones.
[ THE JAB ]
Just a jab, your most reliable tool.
First check the ranges, both the horizontal range, and the Z Range. If both are good, then you have an excellent tool since you will be able to out-prioritize enemies and let them walk into your jab. It can potentially even be broken. Jabs with great Z & X ranges make for powerful crowd control tools, cause grouping enemies will be heavily rewarded.
Next is speed - how fast can you jab with turbo on? Can the enemies squeeze in attacks in between your jabs? Can you safely hit enemies who are doing run-attacks? Sometimes you will pause on impact for a few frames and this can screw with the natural frame data of the move, so watch out for that.
Then check how much hitstun your jab does - is the enemy stunned for long enough that you can approach for a throw, or set up other attacks? Games like Streets of Rage 2 & 4 have very long hitstun and let you use jabs to set up throws, or keep re-jabbing enemies until they're dead.
If your jab is slow but long (grapplers often have these) you should make sure you're further away from enemies when jabbing, let them walk into your jabs. If your jabs are slow & short then you're fucked, use other moves more. Don't underestimate the power of chaining fast single jabs if games let you, they're low commitment and keep enemies stunned, potentially forever!
Occasionally you'll get a context-sensitive attack, such as Double Dragon's uppercut on stunned enemies.
[ AUTOCOMBO/JAB STRING ]
Just your good ol', trustworthy mash combo.
Usually your autocombo only gets performed on hit, but in some games you can continue the combo without hitting anyone. If that's the case, check how powerful the finisher is, cause you might be in for a Bayonetta style situation where you can start the combo out of range just to get the finisher.
First check for variations - is holding up, down or forward during your combo gonna change your last hit's properties? Sometimes, holding up/down lets you combo into a throw, which allows for more aggressive crowd control. Then check it for cancels - can you interrupt it by waiting, turning around, guarding, dashing, etc? If so, the game might have an infinite in it. If your autocombo does less damage than the infinite, it might not be worth using outside of niche situations. Also check if you can mash your combo next to a crowd of knocked down enemies - will you be able to beat them safely, or will one of them sneak past your hits? The last hit of your combo almost always has a lot of recovery, so watch out for enemies punishing it. However some games (Ninja Warriors Again, Sailor Moon) give the last hit of your combo invincibility frames, so be on the lookout for that.
Some games like Fight N Rage have autocombos that move you. This might make it harder to keep your position, lets you be evasive while attacking and encourages constant movement.
Since beat 'em ups have a very dense concentration of kusoge juices, sometimes your autocombo will have poorly balanced range, so hitting an enemy with the first hit won't guarantee that your last hit will connect. It is what it is.
If games let you combo into throws then that's probably what you'll be relying on most of the time. If not but it does decent damage & can hit clumped up enemies well, then prepare to mash!
[ CHARGED ATTACKS ]
If a game has charged attacks - spam them. If they're not great, you'll find out the hard way. Simple as.
[ THE GRAB ]
To grab an enemy, usually you simply walk into them. Sometimes this requires a pre-requisite such as putting enemies in the groggy state, and some games have a dedicated grab button. Occasionally you'll get games that let you grab enemies off the ground like Violent Storm and Punisher, and that's always fun and leads to fun loops.
The first order of business is checking whether or not enemies become ungrabbable when they attack (usually they just get invincibility frames mid attack) - if so, grabs will be a lot more risky and possibly will require set up in the form of a jab or jump attack. The second thing you should try is seeing how long you can hold enemies, in some games enemies will break out near-instantly, in others you can hold them forever. Grappler characters can often carry enemies.
There's also often a variety of grab attacks, from your neutral combo, to special directional ones. Grab attacks can either be paired with the enemy you're attacking, or they can spawn actual hitboxes that hit every enemy nearby, so be on the lookout for that
Sometimes there's a Streets of Rage esque vault that lets you change direction (sometimes with iframes!), and other stuff. Play around with them. Grab attacks, on rare occasions (Kunoichi's down+punch grab in Ninja Saviors), give you iframes, so pay attention to that as well.
Check to see if you can cancel the grab by say walking backwards, or performing another action. This can lead to some nice re-grab infinites.
If enemies aren't good at escaping grabs, don't be afraid to grab an enemy and stand around with them until other enemies line themselves up just right, or to dodge an attack. The enemies are your resources, so you don't immediately have to do an attack or throw.
[ THE THROW ]
The GOAT, a key element of beat 'em ups!
Always check how many iframes throws give you. Most games give you full iframes during the duration of the throw, and many give you a ton of iframes after you exit the throw state. You can use that short period of invincibility to set up even more throws, or do other stuff.
If throws have iframes, then they're a great defensive move - grab & throw an enemy if you feel like the situation's getting hairy and you're surrounded. Or combo -> throw an enemy, if the game lets you (Fight N Rage, Cadillacs & Dinos, Final Fight). You can use throws as makeshift dodges of explosions/projectiles too.
Check whether thrown enemies knock down other enemies, if so the throws are a great crowd control tool. Also check throw damage - sometimes it's better than that of Grab Attacks, so you should always go for throws.
There can be a variety of throws - usually they just depend on which direction you're pressing during your throw, and whether or not you're jumping, so mess around with that to see what you can find. Jump throws tend to feel & look cool, but they are rarely that good since they have a lot of recovery frames and not enough damage to justify the commitment. When that's not the case such as Punisher or Streets of Rage 2 Max the jump-throws own.
Pay attention to the throw's distance & arc - some throws are short ranged slams that do more damage and perhaps have splash-style hitboxes but aren't that good for crowd control. Some throws are long ranged and are great crowd control tools at the expense of damage. Some throws are so long range that they fucking suck.
[ RUN ATTACKS ]
Run attacks have A LOT of potential to be busted in bmups.
Always check how spammable run attacks are - they tend to have huge hitboxes, let you move while doing them, they do good damage, sometimes even extend a run attack into a combo and lack many recovery frames. If a game doesn't have strict limitations on run attacks, it's probably gonna be an OP move. Run left to right spamming attacks without stopping to fight enemies, and it's ogre.
If they're not OP, then they're kind of "get out of here" moves - something you use to put more distance between you & the enemy since they're a bit too close to you.
Sometimes there are varieties of "run" attacks, like Schaefer's dash attack in Alien vs Predator which stuns enemies but doesn't knock them down - they are used similarly to jumps, as ways to approach enemies before you deliver the pain.
[ JUMP ]
This is rare, but some games like Streets of Rage 2 & 4 have safe hitboxes or iframes on your neutral jump. So they can be used as dodges. Sometimes pressing backwards right after pressing jump lets you do a back-jump, and get access to forward jump attacks while traveling backwards. Sometimes jumping is faster than normal movement as well.
[ JUMP-IN ATTACKS ]
Jump -> down + punch attacks usually. These attacks are designed for jump-ins. They have a short range and usually stun enemies without knocking them down. These are great for setting up throws, especially if enemies become invincibile to throws when they attack. Also great against blocking enemies and such. You can often cancel various jump attacks into these attacks.
They tend to be great against running enemies, so if someone like Galsia is giving you trouble, give these a shot
Occasionally they can hit enemies on the ground such as in Tower of Doom, but this isn't that common.
[ FORWARD JUMP ATTACKS ]
Your standard jump attacks - you jump in a direction & press punch. These attacks almost always knock enemies down, and as a result they are similar to run attacks and could be similarly exploitable. If an enemy's too far for your combos to reach, but too close for comfort, do a jump attack to put more distance between them and you, and buy yourself some time. They tend to have high priority too, so they're very safe from range.
Some games have dive kicks, if that exists then GG you've won the game, just use the shit outta them.
Some games let you back-jump, so these attacks can be extra safe since you can be moving backwards while planting a huge hitbox in front of you.
Occasionally useful as a way to get out of sticky situations.
[ UPPERCUT/DRAGON PUNCH/DOWN UP ATTACK ]
Down -> up -> punch is usually how you activate these bad boys. The beat 'em equivalent of dragon punches, including startup invincibility (sometimes). They can work as makeshift dodges (especially since they can often cancel your autocombo) or extensions to your combos. Rarely ever useful in neutral though, and rarely that good for wakeup. Also a good anti-air tool, when that's a factor.
[ DESPERATION ATTACK/SUPERJOY/DEFENSIVE SPECIAL ]
Another central pillar of beat 'em ups. Activated by pressing jump + punch at the same time, though sometimes it has a dedicated button. It's fully invincible, and usually bypasses Z axis checks/has a very wide hitbox, hitting everything around you.
It's your tool for getting out of tricky situations, such as when you're surrounded or being combo'd. Got hit once and lost a bit of health? Desperation attack to minimize damage! See an enemy behind you while you're comboing a dude? Desperation attack! See a dude do a run attack at a range too close for comfort? Desperation attack! See a projectile? Desperation attack! Getting nervous for literally any reason what-so-ever? DESPERATION ATTACK!
First thing you should check is whether or not they spend health on activation, or only when they hit an enemy. If they spend health only on hit, then you can use them as both dodges (projectiles, explosions, attacks which make enemies invincible) and set up tools. For example if an enemy has an invincible wakeup attack, you can time your desperation attack to avoid it and be in perfect range to grab them right as their iframes run out.
Many games tie them to a separate meter, meaning you should use them as soon as your butt cheeks starting to clench without any hesitation.
After that you should check how much HP the attacks eat up - if it's too much then they probably won't be that reliable. Final Fight has a very useful desperation attack because you can do something crazy like 20 of them from full HP. Other games increase the health cost and make them pretty useless outside of escaping failure states. Sometimes they do enough damage to justify themselves in spite of the health cost, such as Knights of the Round's desperation attacks.
Sometimes you can do a desperation attack from a grab, usually some kind of spin & throw. These can be really damn good, even multihitting enemies if say they're up against a wall.
Last thing you should check is what these attacks can cancel - in older games you could only activate them from neutral or hitstun. In newer ones, they let you cancel almost everything with them besides maybe run attacks.
[ SHMUP BOMB ]
Some games have what are essentially shmup bombs - screen clearing attacks that are tied to a very limited resource. Examples are magic in Golden Axe or the cop car in SOR1. If games have these, then it might be better to incorporate them into your stage routes instead of using them on reaction - some screens aren't worth the trouble.
[ GROUND ATTACK ]
*Attacks that let you hit enemies lying on the ground. These attacks fall into 3 general categories :
Pin Down Attacks - You get on top of an enemy and punch the shit outta them. These attacks are very risky and have no real hitboxes, so they're just about doing more damage to a single enemy.
Stomp - You walk up to an enemy and do a ground attack. Sometimes this has real hitboxes and hits any enemy nearby, or multiple enemies on the ground. Sometimes instead of a stomp, jump attacks have this property such as Tower of Doom's down-stab. In some games, hits extend their "lying" state. Usually though, the enemies will instantly wake up after a certain number of hits or a certain period of time passes, so watch out.
OTG Attacks - Normal or special ground attacks that also have the property of hitting enemies lying on the ground. Can be super distinct because they'll put enemies in a juggle or standing hitstun state.
You are basically never invincible during these, and enemies tend to wake up after a certain number of hits, so be careful. Sadly not as common in the genre as I'd like.
[ ROLL ]
Double tapping up or down will often make your character do a side-roll or side-hop. This can be useful defensively, not only because it repositions your character, but also because these moves tend to have iframes. Try rolling out of your autocombo if you're feeling like you're about to get fucked up, it can possibly be a nice mini-reset!
There are all kinds of other attacks in games, and some games have a very unique "meta" to them, but hopefully this will give you a really nice starting point from which you can appreciate the uniqueness of some titles.