So I guess these "gimmick games" SHOULD be short. Like Portal. A bunch of tutorial levels to explain the one gameplay mechanic used, a bunch of "actual" levels where you tightly pack every single set piece you can think using your gimmick, climax, credits. Sure, people might complain that it's too short, but that's far better than people complaining that the game is too long. "Too short" implies that they, on the whole, ENJOYED the experience and would have liked for it to continue longer and will buy a sequel if you make one.
This, more or less. In the case of Mirror's Edge, I'm a little biased because I didn't stick in for the whole ride, and the parts I hated most were apparently at the beginning (eye searing blue skies and reflections, platform puzzle solving, looking for jumps and dying dozens of times before you realize that's not the way you're supposed to go.)
But yes. Single concept games either need to be short, or they need to support that single concept with something. Take Limbo for example. It's pretty single concept. But it's short, priced as such and you sort of give license to a game like that to be short, and punchy.
Mirror's Edge, at that price, with that tech, could have been a lot more and it wasn't. There were some small touches (like with SoC) that would have sold me more on the game. But the repetition and, admittedly, dying a whole bunch just to get the timing right really didn't make for a game.
So what would ME have to have been like for me to like it? If you could have kitted yourself out before missions. If there was more to each level than just where you NEEDED to go. More variety earlier on, in both visuals and game play. (I was annoyed that I had to wait so long for anything combat related...and other than some of the moves, the whole shooting thing felt rather tacked on by that point.)
If I'm going to get frustrated dying doing platforming puzzles, there really should be something in there for me to make it worth my time other than "Finally, I got the timing right." Maybe that just makes me not a fan of platformers...but I've enjoyed plenty, as long as they're backed up by something.