Very well organized forums - neither too complex nor too broad - plus good moderators plus well-categorized accessible tutorials are probably the most important things to exist. The Org has this down fairly well though perhaps the tutorials need to be more noticeable.

Modders tend to help each other out on forums more when the game is first out but as time goes on the pressures of completing mods increase and the good modders cannot spend so long answering questions etc. So when M2TW comes out I suspect we will see more involvement from the more experienced as well as the stampede of new would-be modders ;)