Obviously this is a tough one to call. If your game is too easy, it's boring. If your game is too hard, it's annoying. In both cases, people won't want to play.
The thing I'm going to whine about at the moment is 'easy' games with special hardcore challenges... some of which seem far, far harder than others.
Take Fizzball. This is a fun simple game with an extra-easy Kid-mode, with enough difficulty in the basic gameplay not to make you feel condescended to, but not very hard at all. Of course, it adds all sorts of extra goals to shoot for - win the level faster, win 5 levels without ever losing a ball, etc, to win trophies. Some of these you'll get automatically as you play, some you'll have to do some extra work for, and some seem downright impossible. 'Win a level without ever bumping a tree'. Well, first, in MOST levels there are animals IN trees and you have to bump the tree to get the animal out, so you can't even attempt this. On ALL levels there are quite a lot of trees scattered about, and trying to win the level without touching a tree is insanely difficult. And even if you find a level that looks like once in a rare while it MIGHT be theoretically possible to win - you'll bang a tree by mistake, and there's no 'restart level' button to try again, so you have to quit the level and select it again from the map, and feel more and more annoyed.
Would it be less annoying if it were a mystery trophy and we didn't know what ridiculously hard thing we had to do in order to win it? Not sure. Maybe this just requires reciting 'It's just a game... It's just a game..." until we calm down a bit more.
Worse offender on this is Peggle. Playing versus the computer on anything other than incredibly-easy difficulty is a NIGHTMARE, so vastly beyond the difficulty of playing the normal game that it hurts to think about. And sitting there watching the computer effortlessly score another 100,000 points ahead of you just makes me want to scream 'You're CHEATING!' My chances of unlocking the last remaining challenges for super mastery points seem slim. (What I've got left - the two 'score a ridiculously high score' challenges and the 'beat the computer at non-easy difficulty' duels.) Scoring a million points sounds a lot more likely than winning the duel.
So - do trophies for incredible achievements reward those who can reach them, or taunt those who can't?