Back to the 3D mario discussion: I don't want to sound like a dick, but Galaxy and Galaxy 2 were far too easy. And I know for a fact that I'm not much (if at all) better at the games than the average gamer: my death count in Galaxy 2 was in the high 300s after all 242 stars, which seems to be about what most people had (though many of those were intentional in order to collect star bits... that really bugged me). But there were really only about 4 or 5 stars in the entire game that really felt challenging (thank god for that final galaxy). I understand why the game has to be easy up to the point where you fight the final boss, but after that, it needs to go all Lost Levels on yo ass. I was bothered to the point of anger by the fact that reviewers constantly mentioned the game's "crushing difficulty" or some such nonsense. Yes, I understand that Mario games have never been the most difficult platformers, but every mainline Mario game up to Galaxy was at least to some degree more difficult. Don't get me wrong, the Galaxy games are two of my favorite games of this generation, but I would have had a much better time with the games if I felt challenged more than a handful of times.
I wish that I could get the feeling of accomplishment I got from completing the missions in GX in a Mario game. And yes, GX is the best racing game of all time, and while I try to not care about sales, I get pretty pissed every time I remember how poorly that game sold. You can talk about your "hardcore gamers" all you want, but in a world where F-Zero GX doesn't come close to selling a quarter-million, that term seems laughable (yeah, this is an overreaction, I know, but I take my Captain Falcon seriously (Forever he will be my hero... and not just only for tonight (lol))).
So yeah, in conclusion, keep those 2D Marios easy so they can keep selling 25 million, and crank up the difficulty on the 3D ones.