Adam Spragg, Hidden in Plain Sight
Nothing, aside from technological limitations of any given platform. A game is a game, no matter which platform it's playing on, as long as the input/ouput isn't too radically different. I kind of disagree with your line of questioning, that microconsoles have some fundamental difference to Xbox and PS3. I've only worked with Ouya, which feels like a "normal" console from a development standpoint.
The Men Who Wear Many Hats, Organ Trail
I think the smaller the system the less time and money you will get from the players. You should design around this idea. I'm sure there can be awesome big games put out there successfully, but that's not what the market is currently primed for. Then again... I might just be saying that because I haven't really heard of any yet. Otherwise, I currently expect the scope of games for these systems to be that of a mobile game but with a controller input and nicer resolution.
Eric Froemling, BombSquad
Aside from having a generally more casual target audience, I think there's also some unique design opportunities with microconsole multiplayer such as interaction with mobile devices, since those will probably be a lot more prevalent than actual controllers. I'm envisioning a situation where I'm at a party and there's a big TV with some simple multiplayer game running on it that I can join simply by pulling out my cell phone. Suddenly the game becomes a fun inclusive activity for anyone standing around; not just something the 4 socially awkward guys in the corner are doing.
E McNeill, Bombball
So far, it looks like microconsoles are establishing a reputation as a home for quirky, experimental titles that may not have the polish that's usually expected elsewhere (see: No Brakes Valet, The Amazing Frog?). It might be the right place to try out your more wild ideas.