Jim Carrey's latest verhicle is an attempt to return to the type of Liar, Liar comedy superhit. Basically an average man with a psychological flaw is 'magically' forced to do exactly the opposite of that flaw, and after initial resistance and ensuing wackiness, he discovers he becomes a far better person because of the experience.
In Liar, Liar he was a conniving lawyer who was forced by his son's birthday wish to continually speak the truth, no matter how embarrassing. I'm certainly no huge fan of the film, but the basic concept is clear, the gimmick through which Carrey's character is forced to change is hokey but has a fairy tale sort of logic which enables the audience to suspend disbelief, and the fact that Carrey has to tell the truth and is magically compelled to do so, makes for some amusing and interesting conflict situations. Plus his job is directly threatened by his new condition, as he's unable to lie for his clients any longer.
In Yes Man, Carrey is a lonely guy who says no to almost everything because he's still not over his divorce. An acquaintance forces him to accompany him to a Yes-seminar, where guru Terence Stamp indoctrinates his followers to say yes to anything - absolutely anything. Carrey is singled out for attention, forced to promise he'll say yes from now on, and makes a covenant with himself to keep doing so. If he doesn't, bad things might happen.
So Carrey starts to say yes to any request, and in general the results are very positive - even negative situations turn out in his favour. Later on, he refuses the sexual attentions of his elderly neighbour, but then immediately has some accidents which convince him of the magical power of the covenant, so he goes back to let her ravish him and gets the best blow-job of his life (yes, it's repulsive, no, it's not remotely funny). Job success, a new love affair (in the shape of the not very lovely Zooey Deschanel) and general happiness follow.
Can you see the problem here?
There's no conflict. In Liar, the 'curse' put Carrey into conflict with everything and everyone. In Yes Man, he's conflicted and unhappy at the beginning - and then he breaks free of his self-imposed prison, and the story basically has nowhere to go. As soon as Carrey starts saying yes to life, his problem is solved.
Blake Snyder's Save The Cat structural model has the first half of the second act provide the Fun and Games, the promise of the premise. The second half has the Bad Guys Closing In. This is a story development you often find in comedies. But in Yes Man, the bad guys don't close in because there are no bad guys.
Sure, there are problems in act 3 when Carrey is temporarily suspected of being a terrorrist, and Zooey discovers he went out with her because he said yes to everything and therefore she decides she can't trust him anymore, and blah blah blah. But these feel totally tacked on, and only pop up in act 3.
Moveover, Stamp and his acolytes come across as a band of dangerous lunatics when first introduced. But once Carrey starts on his yes-voyage, they disappear from the movie. And when we finally meet Stamp again, he's just there to impart a wise life lesson (say yes but not automatically, because you want to). So a character who is introduced as a sinister, manipulative huckster is actually a real mentor. Which is just a cop-out, really.
Finally, there's the nature of the 'yes'-compulsion. Whenever Carrey says no (only twice in the film after he starts saying yes), bad things happen to him. So it is suggested the 'covenant' has real power. But in the end, we and Carrey discover there is no covenant, it's just something Stamp made up on the spur of the moment. So Carrey could have stopped any time he wanted to, really.
So, a lack of central conflict, a change in the lead character which solves his problem after barely half an hour, non-organic plot complications just so there can be a crisis and a central joke mechanism which is fundamentally flawed and needs a lot of forcing to keep working - all of these add up to a comedy which just doesn't work. The biggest question is - why didn't anyone solve these problems in the script stage?