Another nice thing other forums usually do is they set up a community feel for their forums. They allow users to add friends and such like that (i.e. like other Nabble users). I think the avatars will help for this, as well (and the new themes).
Plus, depending on the number of posts the users have made (and maybe other criteria, as well: I might say posts that are not spam, for Nabble), they obtain various ranks. Take a look at how Lemma Soft works for that; it's pretty nice for the atmosphere, I think. It would be nice if forum owners could customize the ranks, though (and child forums would inherit from parents, as with the themes), although a set of Nabble defaults would be great. The ranks could be analogous to ranks in a clan in a MUD, too. For instance, a technical forum might have different ranks than another forum. Of course, your rank would change from forum to forum (except for the Nabble-wide rank).
Hmm, it seems it might be good to have a Nabble-wide rank, as well as a rank for a particular forum (but list the more specific rank if that particular forum has specified ranks names). This would be pretty easy to do, complex as it might sound. It would definitely could the atmosphere more aesthetically pleasing, methinks (though in more of a mental fashion than a visual fashion). All you would need to do is use some algorithm to count posts and assign the rank for the forum and for Nabble.
I suggest making the ranks inherited from the parent, unless they are specifically specified in a child.