Besides allowing HTML for Admin usergroups (or any usergroups) the new "Photo Posts" feature where you can post a gallery of photos into a post or make threads just for photos is new and different.
A lot of customizing is now done via the "front end" through the page editor rather than needing to go into the back end to make all changes.
It's very hard to compare VB4 to VB5 actually - they are different on some fundamental levels.
Suggest you play with the VB5 demo:
http://www.vbulletin.com/vb5demo