The notion of "classic tale" has been mentioned several times in these discussions, and I think this is a key idea that perhaps deserves more discussion. I haven't yet formed a definite opinion on this, so I'll just throw out a few observations and hope that someone will pick them up and run with them.
Let's think about some of the classic tales that have really worked: Cinderella, Snow White, Beauty & the Beast. They may be somewhat formulaic, but you can't deny their success. Does it have anything to do with people liking animated versions of things they already know? Atlantis and Lilo & Stitch were definitely unfamiliar stories. Did that unfamiliarity hurt them in any way?
People seem to have strong opinions about Tarzan. Some give it thumbs up and some thumbs down. To my mind, the key difference between Tarzan and Treasure Planet is that Tarzan kept its original setting: the jungle. While TP did some interesting things to try to maintain the seafaring ideas, that just didn't work for me. Tarzan wouldn't have worked for me either, if it had been presented as "human family gets marooned on alien planet - parents die and child is raised by three-headed creature." Think also about Oliver & Company. Yet another classic tale taken out of its original context, and yet another flop.
Along those lines, just how much can you twist a classic tale? When will it work and when will it not? I'm not familiar with the story on which the Little Mermaid was based, but a friend told me that in the "real" story, the mermaid dies at the end. In Victor Hugo's book, doesn't Quasimodo die at the end? Here are two cases where Disney drastically changed the classic tale to ensure a happy ending. In my experience, most people feel better about the Little Mermaid ending than the Hunchback of Notre Dame ending.
I've heard several people express concerns about the fact that the last three animated features were all PG rated. Regardless of the reasons for the rating, in the minds in many people, "Rated PG" and "Disney animated feature" just don't go together. Maybe that has something to do with the success or failure of the films.
Well, that's a whole lot of random typing for this hour of the night, but maybe it will give us more things for discussion.