[MrWookie]

I think the AJAX thing you did is probably the biggest immediate wow-er.

In vB, Ryan implemented a mod where, when on a forum's main page, every thread has a [+] button that, if clicked, shows the first post in that thread w/o having to open the thread. How cool is that?

Other nice perks:

WYSIWYG post making/editing.

The ability to easily quote multiple posts in your reply.

Reduced spam.

A badass [spoiler] tag that hides spoiler text or HH results until teh curious person clicks a button. No need for white text, and it'll be hidden regardless of your skin choice.