Ql: Exactly what I meant to say. Obviously, all non-trivial rpg play challenges the player, not just the character. If it doesn't, the player might as well not be playing at all; if your decisions don't matter, frankly, who cares? (ok, in a very story+ game you might be challenged to entertain and come up with story elements--or challenged to make moral judgments, or frame valid conflicts. But it's -still- all about the player!)
Of course, the other side of the coin is that one of the main purposes of mechanics is to aid us in playing a character with different capabilities than we have. Some play (particularly playing a character less intelligent, or who just thinks differently than we do) can be handled by roleplay and putting yourself in the right state, but it's more or less impossible to play someone smarter than you are without mechanics (or at least narration) giving a little.
But even so, all functional play involves the player making decisions (and deductions, etc) that matter. So just as the player should have opportunities to make tactical decisions, rather than leaving all tactics to rolls on the dice or the GM moving the minis around based on what the GM thinks the characters would do (sounds fun, no? <eg>) , it's much better to have places where the players can make moral and mental decisions than have everything decided by insight rolls and thievery.
That said, particularly when the characters -are- very different in capabilities than the players, it's good to handicap (positive or negative) things a bit so that players playing an insightful character have more information to make a character judgement, or a tricky character are more likely to be believed with a plausible lie. But while the character abilities should have an impact, it's still better to put a lot of weight on player actions--that's what they're there for, after all.