The new version of vBulletin was released as a public beta a few days ago, so I've been playing with it extensively, and for lack of a better word, it's just flat out awesome.
Plug-In Architecture Now administrators don't need to install hacks by editing PHP files. Instead there is a full plug-in architecture. This will be painful for heavily modded forums. I spent 10 or so hours converting my customizations to plug-ins, but it was totally worth it IMO.
Data API You don't have to worry about validating data when custom stuff any longer because you can pass data to the data manager class and it handles all that stuff for you.
Inline Moderation For moderators, you can now moderate inline, managing multiple things (posts, threads, etc.) at once. For example you could merge 10 threads into a single thread with one click.
Template History You can save different revisions of templates (sort of like CVS), with the ability to compare them side by side with an easy to use difference tool that shows line by line differences.
rel="nofollow" Lots of use of rel="nofollow" now, which is a very good thing IMO. Should cut out most of the duplicate content as well as stuff (like reply to thread pages) that anonymous users don't have access to.
Datastore For frequently accessed data (like the list of forums), you can now throw them into shared memory on the web server, so no need to make queries to the DB server each time. Not only is this really cool, but the way it was all setup (with a class and sub-classes) is cool. I wanted support to use eAccelerator as the shared memory system on my setup, and it took no more than 3 minutes to create a new sub-class for it.
Since most of the big SEO forums use vBulletin, hopefully they are all about to get a whole lot cooler. I've already moved the Digital Point forums to 3.5 beta 1, but it's definitely beta software. I've personally already submitted 7 bugs so far (it would have been more, but some others beats me to it).