Just plain fun!
You want to keep playing and if there's any gauge for a game, the desire to keep playing trumps any faults the game has. And it does have a couple. Being limited to 15 of each item can lead to frustrating boss battles. Not being able to synthesize simple items was also disappointing.
I can't help but grin each time i play. The characters are memorable, and thankfully only a few are annoying (mostly minor characters). The humor in the game is perfect. Funny for kids, but mature enough for an adult such as myself to still enjoy. Overall, Tales of Vesperia is just plain fun.