Good things:

- Mission based structure meant more freedom to do what you wanted, you weren't tied down to one path

- Dresssphere/garment grid system was new, refreshing and fun, made a change, and you could experiment with various combinations of characters, etc, plus you got a good feeling when you won a new dresssphere =)

- The game had a lot of funny bits, as was mentioned earlier, and a lot of character interaction just makes you smile, Brother especially. xD

- A lot of the music I quite liked, and got stuck in my head for ages.

- Battle system being returned to ATB made everything more fast paced again after FFX, and being able to get combos, and timing your attacks was a lot of fun.

- The development of characters was good - Paine especially.

- Baralai and Gippal were just HOT. Yes I'm shallow.

- Fun extra games - digging, Sphere Break which wasn't half bad, unfortunately I still can't class Blitzball as fun, but never mind.

- New Game Plus - chance for me to see the ending I want easier!!!

I enjoyed many aspects of the game, especially being able to go back to all the places from X, was like a "trip down memory lane" or something.

However for some reason the game didn't totally satisfy me, something wasn't quite right, maybe the fact that I felt the whole Shuyin-Lenne story didn't pick up till really late on, maybe because I wasn't happy with the ending I got (although I did only get 77%). Maybe the lack of normal battle music, and chocobo music, silly things like when you go to use an Antidote or something it always defaults to Yuna.
I don't know, it's fun, but I still like FFX better.