That wasn't really Adventure's problem. The core gameplay, in essence, was pretty good. The problem was all in execution, which wasn't apparent to me at the time, but becomes bleedingly obvious the more I go back to it. It's virtually impossible to go in a straight line without wobbling back and forth, and whatever momentum-based physics exist in the game are largely broken by how god-awful the collision detection is. The simple act of stopping to talk to people or pick up objects is busted by their insistence of combining that button with the Spin Dash button, and then not registering that I'm ready to talk to somebody unless I'm stepping on their toes AND facing directly towards them. And don't even get me started on the horrendous camera... Not even DX's "free camera" (which is inverted on both axes from what I'd like and has no option to change it, argh) is all that good.
Sonic living in a human-filled city? Doesn't bother me that much, actually. Sure, could've used a few extra furries for consistency's sake, but eh. Also kinda miss those weird floating TV things - the ones that apparently talk to you - that showed up in Adventure and never once since.