I like the classic forum view. I think it might be nice to make the defaults for before you register switch to something like this (or at least one of the new views), seeing as most people expect something of this sort.
Now, although I like this, I'm not a huge fan of the way other forums tend to outline topics (i.e. a flat page without seeing the parent-child thing going on). However, I do think the aesthetics of the Nabble method could be improved; I'm not sure how, quite, though.
Also, on the topic of how other forums do things, are you planning to implement polls? I really like how they do this at http://lemmasoft.renai.us/forums/ though I might suggest also letting people use checkboxes (instead of just radio buttons), as these would be rather useful for cross genre polls and such. For instance, someone wants to see what people think on a subject; they make a post with a poll included; people who view the post will see the poll there above the message and will be able to take it; any user can see the results of the poll. The poster could determine how long the poll would go for (or whether it would go indefinitely, or until stopped by the poster).