The biggest flaw in X-2 was that it even happened. FFX was the first ever Final Fantasy game that had a sequel, as well as the first Final Fantasy game that had actual talking characters. Well, since the major milestones had been passed, what was left for FFX-2? Nothing. Don't get me wrong, it was fun to play (the first couple of times) and had a great battle system, but there was really nothing major that set it apart from FFX (except that X had a better storyline). Final Fantasy has always, for as long as I can remember, had a main character with a love interest. So that didn't really make X-2 stand out either. Yuna lost Tidus and then in the end they were reunited. Ta-da. You can tune into lifetime and see crap like that.
I know, I used to loooove and swear by X-2. But I've grown up a lot and moved on from the sappy storyline that entertained me when I was 14/15.