The animation was beautiful, the writing was clever but not too self aware, and the humor was brilliant. They managed the subtle humor that the adults would understand coupled perfectly with the funny 'falling down bits' that the children would love in a way Shrek could only dream of, without sex or potty humor! [And you said it couldn't be done!] In addition to that, there was a lovely and very meaningful message of: This is what happens when we rely solely on technology, and we don't take care of our planet. And the best part is that the film did not beat you over the head with it which only enhanced the intricacy of the writing and the movie as a whole.
It was this balance of subtle and slapstick humor that allowed the creators to craft a film where the first fifteen minutes or so had no dialogue whatsoever [or at least any we could understand] and that just made it that much more interesting! Also the fact that the 'bad guy' was not really a bad guy and was kept that way by clever use of circumstances and how robots work made me appreciate it that much more. Don't get me wrong, I love a good fairy tale with the hero and the 'evil' sorcerer [or whomever] that is defeated and the day is saved, but this was refreshingly different.
I will keep this short because I could gush all day about this movie and every little bit that I like, but I will say that not only is Wall-E a great movie, with love, humor and danger but it is an important movie. It is important not only in the message that it is sending, but the way in which it was made to send that message.
Take note writers, artists, animators, filmmakers, this is something to strive for!