What is this? What makes you think you can submit a site for review
where both the (X)HTML and the CSS actually validate?
Is this your site? If so, why does it have Philipp Lenssen's name on it?
And if it isn't, why are you asking?
Anyway, assuming it is actually your site:
I quite like the look of it, though the colour scheme seems a bit bland
to me. Perhaps that is the intent. Actually, it is unclear what the
purpose of the site itself is - you need to state that clearly on the
front page.
Horizontal scrollbar at full screen 1024x768. (Moz1.6)
Don't specify font sizes in px or pt - users of IE (and NS4) can't
resize the text to something reasonable.
The text doesn't fit over the background images properly once font size
is increased a bit, but I guess you probably expected that.
The CSS file is pretty big - it could definitely be reduced in size, as
there are at least a few styles that are defined more than once,
including some that contradict each other.