Ease of use overall. I participate on a couple other boards that are not UBBT and they work okay but are not as easy overall and esthetically pleasing as UBBT
The other boards do not have an easy if at all way to ever converse with someone actually doing the coding. You can talk to people that are Tech Support in the forum and they will "pass the information along" - (yeah right), but here you can actually get a response typically in a very short time from the developer.
That alone is a BIG difference worth it to myself and many. It gives you more of a feeling of (I belong to a group that does want to share and help each other rather than I'm just a customer).