I've seen too many other forums that were outright hostile to users. (One reason why I moved from Python to Ruby for scripting was because the
#python IRC channel was condescending and rude to people asking questions;
#ruby denizens were friendly. One was a nicer place to be than the other. I knew which group I wanted to ask for help when I needed it, so I chose my tools accordingly.)
After that experience, I've paid careful attention to the mood of online communities. My theory is that the initial group of Stack Overflow users were civil-minded and helpful and they passed on to us the expectation that we should all remain civil-minded and helpful even in disagreement.
It is amazing that the culture from the start has survived. When companies or communities grow too quickly, there is often threat that whatever traits made the company or community awesome in the first place isn't sufficiently transferred to the new hires / new members. Perhaps the separation of "content" from "meta content" between the main site and meta site is core to the civility -- disagreements there are confined to technical issues and source code / reference to standards carries a lot of weight. Personality conflicts don't show much in the technical portion, which is where most of the real traffic is, so the distractions of management or style or control or direction or whatever else disrupts so many groups is always on the sidelines of the main event.
We have a few 'prickly' users, but by and large, rudeness is so far outside the norm that people show up to learn something specific, good behavior is rewarded, and hoooray! it's still a nice place to be.