What I like about Ask Ubuntu is the way that It's built and run by the users. This means that the site is for users by users. When you are thinking of a feature request its most likely that someone shares the same view. Also the dedicated guys who toil everyday in the close votes queue making sure substandard questions and spam don't make it onto the site.
When I first switched (partially) to Ubuntu I visited the forums and I did not like it. Don't get me wrong, Ubuntu Forums are an integral part of the Ubuntu Community but, its not just my thing. I was having a problem with wireless on Natty and I had looked through many solutions but couldn't solve the problem.
By chance or good luck I ended up on AU, to cut a long story short I found an answer that solved my problem (something about ndiswrapper) of course I cannot remember the solution now :) but I've never left AU since although I have been a passive user. I love that people are willing to help and they try as much as they can not to be rude and maintain patience.
Also I like that your experience is not judged by your age (lots of teens here know so much it surprises me). Lastly and according to me the best reason for AU's existence is the feeling of community, the feeling you get when your answer is picked as the answer by the OP and the feeling that you are helping the community especially when they comment "thanks" (though highly frowned upon) and give upvotes.