clicky I think for me, aside from the nostalgia of web-2.0-style forums, is perhaps the greater information density. With the old forum, I'd argue that it was much easier to see everything Lewd had to offer, neatly categorised. Whereas here, while visually the forum may be more appealing, it feels like all of the threads are just shoved into one bucket, they take up a lot of visual space, and if they're not on the first couple of scrolls, forget about it. Any thread behind the "Load More" button may as well not exist. Sure, we have categories here, but they're more like tags than proper sections.
However, if I had to pick only one thing that I'd improve about the current forum, it would be the thread icons. Currently, each thread has the avatar of its OP set as its image. I think it would make the site feel more dynamic if it showed the last poster's avatar instead.
