I am not a big fan of the entry page, it really offers nothing to the visitor. The image is nice, it just does not give much else outside of that.
The logo is very sharp, and I like it a lot. The odd spelling of Architekt seems a little out of place, but it is eye catching and will stick in the visitors mind.
It is obvious that you spent a lot of time making the forum look like anything but a forum, and in this you did a great job. Until I actually got down to the post level I found it very hard to identify it as a forum, and even there it is very custom. It is a very clean, if somewhat simple look. The actual forum posts do seem a bit out of place next to the rest of the site as it contains more color variations. You could adjust the colors here a bit to really tie everything together.
So far it looks really great. I can see where some people may think it is too simple or dull. I do not agree with that, personally I find the look very crisp and easy to navigate. You have done a great job so far