The most unwelcoming forum in the world is, without a doubt, FacePunch. A friend of mine recently posted a map on there and nearly started a cross-community flamewar... because some boxes didn't have curved edges. Scroll down any thread and it's almost compulsory that one of the people on there will have been banned.
These forums are definitely the most welcoming... it's quite possibly the first one where saying "hi" actually merits a reply.
"N00b" or "noob" is a more derogatory term than "newb". In Counter-Strike, calling someone a noob is a downright insult. But calling someone a newb just means that nobody expects you to aim properly
. I've had experience.