Stack Exchange may not be a threat, but the up and coming Discourse, Vanilla, and countless others certainly are.
<flame>The experience of using most forum software makes me want to puke, and then go back and find the poor unfortunate soul that had the idea that forums on the web were somehow better than Usenet, and boil them alive for their crimes against humanity.</flame>
Most forum software out there is horrible painful to use. You have to scroll and scroll and scroll and page and page and page and page and click and click and click and click, and then you have to do all of that over and over again because you posted a message or clicked back to the index when you wanted to go back to the topic, or weren't looking at the right post. A moderately busy "traditional" web forum will take you hours each day if you are actually trying to keep up with everything, and attempts to use the "unread posts" feature are frustrated by an endless sea of "Me too!!!!" posts. It's no wonder that many forums get taken over by the trolls and spammers - they tend to be the only ones persistent enough to stick around in the long term.
Now as for competition, Discourse (http://www.discourse.org/
) is forum software created by the same people as Stack Exchange, under the same principles. It builds on Stack Exchange and applies it to a discussion format rather than a Q&A format. All the "oh, that's Q&A we're forums" stuff I saw in this thread, well, watch out, because Discourse shows how the same principles can and do apply to forums.
Almost as important as what it has is what it doesn't have (http://www.codinghorror.com/blog/2008/0 ... loper.html
) (http://meta.discourse.org/t/what-is-wro ... 63/best_of
). The user interface is very clean, and makes good use of the space. "Categories" are emphasized in favor of "Boards", and by default, the first thing you see is a list of all the active topics, regardless of which category they are in. Topics are at the top level of the site, and categories are something you can filter by, rather than boards being at the top and having to drill down into each one to see what's being said. Takes some getting used to, but it makes it easier to digest the information and find what's interesting - once you use it for a while and go back to having to visit each of two dozen topics, you'll see what I mean.
Another thing Discourse does right is getting rid of the paging. There are no pages to a topic, to matter how many posts it gets. Instead, as you scroll, more data is loaded via AJAX requests in the background for an endless scroll effect. This is less jarring than having to click back and forth between pages of anywhere from 10-50 topics or posts each. Small but important win.
As for how "social" features can apply, you have reputation systems, which are being implemented along the same model as Stack Exchange to gradually give people with good reputation access to moderator tools, and gradually push people with bad reputation away. You have social login almost right out of the box, so you don't need yet another account for every forum, you have real threading and quoting to maintain context, a "best of" view on busy thread's that's powered by the "like" button, bookmarking, @mentions, etc. All of this feels like it's a natural extension of the software rather than "oh, that looked cool, let's add it"
Already, in it's infancy, the interface shows a lot more polish -small details like side by side previews in the editor are appreciated, as is the post editor appearing right there in the page, with the ability to scroll through the other posts and quote them as you are composing your own - even as they continue to be posted..
It's also worth pointing out here, Discourse and StackExchange are also very purposely built to promote a certain cultural ideal, that of intelligent, insightful, and efficient discussion. By having the culture and code work hand in hand, the signal-to-noise ratio on these sites tend to be orders of magnitude better than just a traditional forum, because they go out of their way to weed out anything that's not constructive, and reward everything that is. Sometimes this is done gently, sometimes it's done very abrasively, but not only do these sites deny trolls a forum, they also deny "me too!" posts, 2 page signatures for one line posts, and many of the other things that seem cool at the time, but in the aggregate, make a forum site a cesspool. (see http://try.discourse.org/faq
Now, I'm sure someone's going to say "well, why don't you use that instead". phpBB used to be the state of the art, awesome software. Now it's kindof stale, but it doesn't have to be - there's still obviously a vibrant, dedicated community that is passionate about phpBB, and that's a good point to start from. There are lots of others out there including bbPress and Vanilla that are nowhere near as stagnant, but there are so many of them that none of them really have the following the phpBB has.