You are right when you say xNN is difficult to hack on. It takes a while to get used to epoptai's style. I keep telling myself I should sit down and refactor it one day.
One of the nice features xNN has, that bivnn yet lacks, is displaying the response nodes in their threaded order underneath a primary node. It's a really effective way of letting you see how busy a node is, long vs. deep, bushy vs. thin and so forth.
Secondly, for new nodes that are replies to older nodes that themselves have fallen of the end of the newest nodes, they get listed in a separate section, with a link to the parent node as well. This lets you jump straight to the parent node, so that you can read the response in context.
I think the default xNN behaviour in this section was to sort the titles alphabetically. Whatever, I didn't find it very useful. I hacked it to display by sorting the parent nodes ascendingly, which means you see the reponses to the oldest nodes first, and multiple new nodes to an old node are grouped together.
The final feature that xNN has is, put it this way, your HTML design skills are about on par with mine (that is, essentially non-existent). epoptai is a grandmaster paramount HTML designer who moved from designing web pages to writing Perl code without any formal training (which goes a long way in explaining why the code is as it is, and why the results look as good as they do).
g r i n d e r