Understand that almost all forums on the internet either use phpBB, vbulletin, or IPB. They all have an extremely similar featureset. There are some other semi-popular boards out there, but they have also have the same featureset as the others. So this wordpress forum will *always* feel unfamiliar and hard to use for any new user. You have basically put up a road block that some new users will be willing to get used to… and some never will be.
Also, realize that the forums will never recover if certain features are not fixed. The main thing that comes to mind is the search functionality. As it is, the search is worthless. Until search works… the forums are almost useless to me.
By the way, your old phpBB forums can be run in a “narrow” format with ads on the side, just like these forums are. I didn’t really understand that comment.