The battle system
Triple Triad
Gunblades
The music
The fact that enemies level up with you, encouraging efficient, low level play and exploitation of the junction system
The Limit Breaks
The cool intro FMV
"Then go talk to a wall"
The Gardens
Garden War
Ragnarok
Ultimecia's Castle and the bosses in it
Doomtrain
Esthar
Odine's collar
The ending