Wow Tusserte, didn't even realize that about the header but I really like the simple look, and felt it had some kind of power, especially the bubbles background. I'm pretty sure I related that to people coming together like in a social network, although it looks more like a cloud to me.
Ajax and other things does slow the page a lot, and I like how you didn't use images for the buttons. CSS can be powerful so a good idea of taking advantage of that. Try using border-radius, it's part of CSS3 but it's getting a good amount of support and degrades well.
I agree, accessibility is a very important point although many people avoid it. One thing I would use though, is instead of leaving empty anchor tags (<a></a>) with no text (I noticed it in some places, not all), try putting the same text that is in the image and use text-indent: -6000em; or something similar. Display block does not work well for SEO, so don't use extra spans and such for that.
Again though, awesome job I do wish to do some more posting around here. Also it's cool to meet you again with a new site, brings back memories but most importantly I can see clearer how far we all came. Imagine, from Freewebs and a site builder!