It's been said before, but the plugins are the best, and on that note, the support of the .org community.
Hmm.. Let's see. I've seen current vBulletin's power intranet sites providing medical charts to doctors around the world. I have seen it power content management style sites with simple template changes to move away from the forum site. I have seen it power websites with 80,000 concurrent members online at the same time. The engine could be used to anything from craigslist type sites to a media site like Youtube without completely rewriting it. It wouldn't do it out of the box but wouldn't require a lot of modification. In fact most of the modifications already exist.
The way its both (almost)perfectly configureable and how east it is to use for the end-user(visitors) and Moderators.
