I'll be honest, and give it its weak points: The main character (Jim) is quite blank and not very well developed. Which is bad because an audience cannot connect to a flat, central character. Secondly, the CG (although very well stylized) I think didn't mesh as well with the 2D animation as it should have. Other than that...you might pick apart a few other things about the movie (and no, the dialouge isn't that bad. it's a G rated movie...it's not supposed to be incredibly complex).
Treasure Planet keeps pretty tight in line with the original classic, "Treasure Island", even when softening it down for a younger audience. I grew up with that story, and had no problem with the adaptation in Planet. Also, this film contains two masterfully animated characters, John Silver and Dr. Dilbert Doppler (animated by two masters, Glen Keane and Sergio Pablos, respectively). If you're an animation enthusiast, these two are gold to watch (especially the pencil tests in the special features on the DVD).
This whole review is a little long-winded, but in closing, this movie is more than what was said of it. If anything dealt the killing blow in this movie failing at the box office, it was everyone's bad word for it. Not true. Please see for yourself!