A couple of small observations. The layout of posts seems to be a bit off in IE6, at least on my computer ... avatars in the middle of the screen, with post text shunted to the right. This may be my computer though, and I doubt anyone will have too much sympathy for someone using IE6 anyway. There are probably other issues for IE6 that you won't be able to do anything about since it isn't supported.
Another thing, about the icons to the left of post titles in any particular forum. I think hovering the pointer over these icons is supposed to bring up little balloons with the status (New Replies, No New Replies, Hot Topic, etc). It does in Explorer, but not in Firefox. Maybe this is my settings, and anyway, it's a very small point since it's pretty easy to work out the meaning of the various black, grey, red boxes, etc. Just thought you might like to know.
You really need an up to date browser for everything to work how it should.