A whole lot of things, but in particular, the streamlining of the interface. You waste so much time waiting through corporate logos and loading time and health warnings and who knows what else before you're able to get your arcade fix. I hate this. I haaaaate this. Playing emulators on my computer has pretty much reduced my tolerance for waiting through bullshit to nothing. El zippo. I just want to play the game, NOW, while the mood strikes me. It's how things were in arcades- you just walk up to the machine you want to play and pop in a coin- and it's how I want things now.
Also, everybody who makes commercial emulators goes nuts with presentation, and no one style really meshes well with another. There's no consistency. I just want to pick the game and play the game, and some of these commercial emulators (case in point: the most recent Sega Genesis collection) actively interfere with you trying to reach the game. No, I don't particularly want to search through a fourteen year old's bedroom for their Sega Genesis. That's creepy and weird. "Officer, it's not how it looks! I just wanted to play Billy's Sega Genesis!"
So yeah, I buy the classic collections whenever I can, to support the companies and to stay at least somewhat legit. I whined for these games to be made legally available, these companies have made that possible, and I suppose I'll have to put my money where my mouth is. It's not where I actually want to play these games, though. Having an all-in-one, no-nonsense solution with consistency in the interface is far preferable. Since that can no longer be the Xbox Series, I'll have to find another solution, and none of them were this convenient.