It will take some time, but the big difference between this and the email-based or Yahoo-based groups is that the information is closed off unless you sign up. Someone who wants to learn or just read can't access things. While some people have websites up with good information, a lot of information gets passed along in discussions of problems.
That being said: I can't speak highly enough of the Messerschmitt Owner's Club Forum which is hosted on Yahoo Groups:
http://autos.groups.yahoo.com/group/schmitt_talk/Signal to noise ratio is VERY high (good thing)
People participate from around the world and are VERY helpful.
I don't think this should "win out" over anything else. I do think there is a need for a publicly accessible Vintage Microcars Discussion area. You don't need to pay anything or participate if you don't want, you can read the posts without "signing in".
I agree about the marque-specific categories as things grow.
These can easily be created and topics moved there.
I think it will be some time before that happens though, unless a few people lobby me for something and want an area for their own particular interests apart from General Discussion. Kei Cars would be a good start.