I've talked about it before but Half-Life is, to oversimplify a bit, the thing that killed the boomer shooter as it existed from 1993 to 1998. It was a (relatively) slower, more tactical shooter with bulkier enemies that encouraged the use of cover to avoid largely undodgable attacks, with level design much more linear and guiding you through a sequence of set pieces, with much less focus on secrets. Mind you, the game still largely falls into the general area of boomer shooters, it has more in common with quake than it does with halo, you still move really fast, there's still a wide variety of enemies with distinct silhouettes, there's a massive arsenal of weapons with distinct uses throughout the game, and its still a game that I hold dearly in my heart, but there is a direct line between all three games and Half-Life is a very clear demarcation between the types of shooters that came before, and the type of shooters that came out after.
Half-Life 2, on the other hand, very much falls into the camp of a post-halo shooter. You move real slow, sprinting is on a regenerating stamina mechanic, the wide array of recognizable enemies has been replaced with different types combine soldiers, zombies, and headcrabs, all enemy attacks are either hitscan or melee, the implicit and largely skippable narrative of Half-Life 1 has been replaced with long sequences of watching characters talk to each other, there is still a wide arsenal of weapons, but it doesn't necessarily always feel like that when so many of the weapons strictly outclass others. And like, none of these things are strictly bad, Half-Life 2 is an iconic and beloved shooter for a reason, but it is extremely obvious that Half-Life came out in 1998, and Half-Life 2 came out in 2004, in a post-Half-Life, post-Halo world.
This is all a long way of saying, purely down to personal opinion, I like the first person shooter genre as it existed in 1997 much more than I like the genre as it existed in 2004, or 2015 for that matter.