Case in point: Doom 3 vs Half Life 2.

While one could certainly argue that id's games haven't changed much over the years in terms of macro gameplay elements (running & shooting), I think Carmack and his cohorts at id are vastly underrated when it comes to micro gameplay design... the art of just making a game feel fun to play.

Sure, you're just running and gunning down a horde of aliens/demons in all of id's games, but doing it is a LOT more fun than in most other games in the genre. A lot of this is actually due to gameplay-via-code and thus attributable pretty directly to Carmack. Turning feels right, movement feels right (not realistic -- right, there's a difference), shooting feels right, jumping feels right. It is amazingly hard to get these micro-gameplay elements to truly feel "right", and few manage to do it as consistently as id has (some other notables here include Nintendo and Bungie).