General suggestion: keep it simple!

I mean, technically simple... The more there is glitter and glamour, javascipt and popups, etc. etc., the more plausible it is that something is broken, or at least that people with older browsers have difficulties (and those are often those less technically savvy, too...). As long as it does not go as far as supporting last millennium browser versions (hmmm. I might have Netscape 4.something installed somewhere... ;-) )

Also simple site scales better (the more glamour and glitter, the more data to be moved,. the more strain on the servers...) - also in the sense for working with setrtings for the disabled (enlarging font sizes, browsers for visually impaired...).

+1 :)
Also, please don't hide things and don't require hovering over things to see other things. That's annoying when it covers other things (like profiles covering messages) and it cannot even function on touch screen devices like smartphones and tablets.

Yes, I want to third the anti-popups thing. It's the only thing on this site I would actually call "bad". I know not very many people are doing NaNo on tablets or phones right now, but that number probably isn't shrinking. That renders hover-overs pretty useless--even if you know they're there, your finger covers what pops up.

And the nature of NaNo dictates that people are going to try to snatch writing time in some weird places--on buses and planes, unfamiliar computers, etc. It's nearly impossible, when people are in a hurry and want to just grab a bit of site/forum time, to lazily troll through every page and hover the (perhaps nonexistent) mouse over every page element.

That's a great way to include easter eggs of extra content for the dedicated site users who have the time and physical stillness to explore, with a mouse, all the bits of a page. And frankly that would be an awesome thing to include.

But it's an awful way to show important information, like the word counts of others or the time stamp of posts. That's just basic stuff, and acquiring it without having to move anything but your eyes is ideal.