Ghibli films just tend to be a bit more dry. By contrast, Disney does humor pretty well, and have probably some of the most memorable musical numbers in cinema history. Again, this probably due to cultural difference.
Another reason for it is that Ghibli movies are supposed to be art films, whereas Disney's first and foremost priority is to entertain. When I say "art", I mean more than just the animation. There's a lot of humanity and raw emotion in their stories, a good example being Princess Mononoke. In a way, it's a story about mankind: a human's need to be the master of his environment (can also be seen in Castle in the Sky), people loathe and even attack what they don't understand, and (apparently) bad things can happen when man tries to separate himself from the very earth that bore him, to the point of maybe even destroying it (in other words, its saying man can and should peacefully coexist with nature). Then in Spirited Away, it's about how courage pushes this young girl past her fears because of her overwhelming desire to see her family again. It's not about an actual plot so much as it is about certain events that happen to her.
So art films try to evoke thought from the viewer, and in these cases, display a mastery of hand drawn animation. Disney, however, wants to entertain you, and that's why I think their films are better on the whole. After all, the main goal of film is to entertain the audience; Disney recognizes this, prioritizes it, and does extremely well as a result.