As a relative newbie, I'd like to offer some feedback relating to my experiences so far.
I have found the site very useful and would have liked to upvote the questions and answers that helped me, but for the seemingly vicious circle of "you can only take part if you have rep, you get rep by taking part".
I eventually had a question that did not appear to have been asked, so I asked it. Very shortly afterwards, someone pointed out it was very similar to two other questions (which I think highlights that the search facility isn't all that brilliant). However, I was asked to leave the question as it was worded better than the others, and I finally got some rep points!
It must have gone to my head, because I then spotted an unanswered question on a subject that I've had a lot of experience in, and jumped in. I thought I was posting a comment but it turned out I'd added an actual answer. Immediate downvote, no explanation, although I guess it was because the question itself turned out to be a duplicate, and a somewhat vague one at that. A mod kindly changed my answer into a comment and explained that I couldn't comment as I didn't have enough rep.
Maybe I should have looked more carefully at what I was doing, but it's certainly made me more cautious about answering any other questions.
So, what do I think of Ask Ubuntu?
I'd say it was generally a nice and friendly place, and certainly more so than some other sites. That's largely based on how I see the users interacting with each other; my own experience is mixed.
Rudeness and belittling language
Can't say I've seen any, although some answers/comments can tend to have a 'being told off by the headmaster' tone to them.
Being welcoming, patient and assuming good intentions
I totally understand why it's in place, but the rep points system does make you feel like an outsider in a member's only club, which is not especially welcoming.
Not being jerks (name-calling, bigotry, inappropriate language, harassment)
Not seen any at all, so 10/10 for that one.
As don.joey has already mentioned, it does require some sort of mental shift to use a site like this properly when you're used to a discussion forum. But that's something for the users to deal with, not the site itself.