Personally, the guild's dark background has always been a nice touch. The white text pops out nicely and I don't have to strain to read anything usually. I've spent many hours writing in the middle of the night. It's nice when my eyes have already adjusted to the dark and the computer screen isn't blindingly white or some other super-bright color. An ergonomic color scheme may seem insignificant, but it has heavily influenced how often I come back to this site. 
Like Krot said, "it's simple". I've been to other sites which assault your eyes with flashy banners and animations, and it often ends up being more of an annoyance after the novelty wears off.

I know "intuitive interface" seems like some marketing buzzword (it really is), but it has its place. I'm much more willing to stick around a site if the pages are organized and easy to navigate, compared to a maze of links and subforums. The less I have to think about navigating to the content I want, the better.