First of all, this is because it's funny. It's not so funny that you're constantly laughing out loud, but there are more than enough good jokes in the movie to keep you entertained, and it's full of good observations about superhero movies (note all the ridiculous superhero and villain names) and references to other movies. It's also a brilliant James Bond spoof/homage, containing all the gadgets, secret island bases and crazy schemes that made early Bond movies so much fun.
Secondly, this movie has heart and charm in droves. It's full of memorably and likable characters you can really identify with and care about. Characters like Edna are instant classics. The voice acting is universally excellent, but Holly Hunter steals the show as Elastigirl. This is a movie that doesn't need toilet humor, characters modeled like celebrities, or a "hip" soundtrack...it easily succeeds on charm and style alone.
Also, I found it to be a surprisingly good action movie. The plentiful action sequences are full of energy, extremely fast-paced, and exciting, and put many a live-action movie to shame.
Finally, I also liked the tone and message of the movie. "The Incredibles" basically makes a plea against mediocrity, breaking with the politically correct idea that "everybody is special" (and thus nobody really is), and encouraging people to use the talents they have been given. I think this couldn't be more true. Not all people are equally talented (just compare Pixar to Dreamworks), but this is no reason to keep the talented people from not reaching their full potential just because it makes the less talented ones feel bad.
"The Incredibles", while maybe not as hilarious as I had hoped (though it's still very funny), is nonetheless an extremely entertaining movie, that manages to charm you wit its combination of great characters, lots of style, tongue-in-cheek attitude and good message. Highly recommended.
****1/2 of ***** stars