The scaling part is definitely the really hard part. Our community is still pretty small. But in any case, it's the early focus that will define the whole community. I think Stack Overflow cares a lot about being more welcoming, but has a big hill to climb in that sense. Hacker News doesn't seem to have any interest in changing and that's also a reflection of the folks behind it, IMO.
Reddit is a complicated beast. It's kind of up to the individual moderators to define the experience and expectations. The company would have a hard time changing things significantly if it were their biggest concern in the world (and it kind of is). Because Reddit has so many dark corners, relatively tame communities like /r/programming seem not worth worrying about.