Couple of reasons come to mind:
1. Threads have comments that are insightful as opposed to 101 trolls on comment threads within other sites. Opinions, counter-points and a bunch of educated users can all add to the discussion value, without any need for unmitigated flaming and knuckle dragging from fanboys
2. No use of signatures; I find to be a boon because they can get bloody enormous on other forums, where I'm not really interested in what irrelevant stats they display about the user
3. Moderators that actually keep the forums clear of any short answers and offensive opinions designed to offend, including locking of threads that deem to go down to wrong path