what makes them good? incredible animation(some of the best 2D animation in animation history, one of reasons I love these movies so much is because they kept it traditional instead of going into 3D like most of others did.)
absolutely amazing(occasionally) songs, outstanding voice acting(Jeff Bennett being my favorite), which, mind you didn't get worse even in the TV series. Decent storylines, not all of sequels of course, but most of them, had an interesting enough story, my favorite being Stone of Cold Fire
Great music. Again - my favorite was in Stone of Cold Fire. I'd even say the sequels pretty much surpassed the original movie. I do wish however they were a little darker, like original movie, movies 7 an 10, and weren't too light-hearted like movies 11-13 were