The problem is that they are using rather primitive forum systems like phpBB and the like.
After using reddit, phpBB is like going to the stone age, imagine if hacker news was on a simple forum system like that.
I know reddit is an aggregator site, but I think they are the strongest competitor to forums, think about it: how hard is it get the gist of a discussion on one of those forums, where you have to scroll through tens of pages on a really popular topic.
my point is: why doesn't reddit create a product that will allow any site to to have their own subreddit style page completely autonomous of reddit.com?
It's easy to do this on reddit itself:
1. Create a subreddit for your new forum.
2. In the subreddit control panel, enter the domain name you're going to use for it.
3. In your domain name registrar control panel, point your www cname at rhs.reddit.com.
4. Voila, you've got your own subreddit style forum, with the benefit of it being integrated into the reddit supercommunity to whatever extent you want.
It's a neat idea but it needs a lot more polish to be really useful.