Few things, I would change:
1. Banner.
There are two major troubles with the banner. First, its width needs to be changed. A banner that forces you to scroll the page horizontally is a bad thing, no exceptions. Not everyone is using a high resolution screen (think about the ever increasing number of netbook users). Second, the banner overwhelms you with many small images, but lacks a distinct and eye catching feature. Personally, I would design a banner based on the BMW colors and maybe add a single image of a car. The forum title inside the banner is WAY too big.
2. Forum icons.
Already mentioned by others - they are too big.
3. General design
a) Hovered tabs are almost unreadable.
b) reconsider the forum structure. The top level seems to make sense, but some of the sub forums don't. For example, E46->M3 - don't expect that such a forum will get much content in the beginning, so I would merge them. Your current forum structure will result in quite a few low activity and low content forums, which isn't the best thing to have. Start with fewer sub forums - you can always change it later when you get more content.
c) Personal opinion, but I would change the overall color theme and make more use of BMW colors. Something that focuses on blue, white and black, throw in some grey or silver tones and maybe some (rare) red elements. Ideally, a visitor should instantly associate the site with BMW, even without actually reading text.