So what exactly is so bad about it?
I'll give you the short, censored version:
The feel of the game is completely different, it's cheesy, there's catch phrases and everyone is now 'cutesy'. Gone is the shy, reserved but subtly beautiful Yuna, replaced by some chick in hot pants with dual wielded pistols. The game is also riddled with stupid minigames that were clearly inserted to please the Japanese anime-kids that wanted to perv over Yuna and Rikku in the original game, one such example is when Yuna has to give a massage to the female villain in her lair... no, i'm not kidding.
The villains also are no longer sinister or epic, they've been replaced by three bumbling comic-relief characters who spend more time tripping over rocks than threatening anyone. There are also many, many cringe worthy moments, one such example is when you have to arrange a concert for Yuna to sing in to cheer her up, believe it or not this section takes up a large portion of the entire storyline. The best characters in the game have been ruined (Yuna) or just don't feature in the game, Wakka and Kilmarhi have cameos but most of their dialogue isn't even voiced. The playable characters are limited to three, two of which (Rikku and Yuna), you already know the back story of and therefore there is little character development. Payne is pretty boring too, basically a carbon copy of Lulu in personality but in a skimpy leather outfit.
The best way to draw a comparison i have always found is to watch the intros of both games which i have included below. Watch the X-2 one at your own peril, have something handy to chew on or else you'll probably gnaw your own hand off.
http://www.youtube.com/watch?v=zqJYXoC1OQc
http://www.youtube.com/watch?v=SyqX1SNuwzc
The one, and i mean the one good thing about X-2 is the 'complete' ending. However to get this you have to win possibly the most annoying and difficult mini-game to ever grace a final fantasy.