Stack Overflow/Exchange is pretty good...the UI doesn't get in the way of doing things. Unfortunately, they have also been making moves away from the content being the most important thing.
The SO UI for participants (rather than people looking for an existing answer) is mediocre-ish to weak. They seem to be a thoughtful lot so I imagine some of it is deliberate but that itself points to more fundamental design problems - making things annoying is better at annoying people than it is at discouraging certain types of user activity.
