I'm going to bump this because of the comments on "why is PlayDip doing better" in regards to new player retention and total users, and I'll offer some outside looking in advice from someone who primarily plays over there.
It comes down to one, very simple thing. They don't just look for "diplomacy players who want to play diplomacy online". They acitvly look for players who never have played the game before, hell never even heard of it, and facilitate actually teaching them the game. Their "mentor" games as they are called fill up fairly rapidly, and involve 7 newbies and a game teacher instructing them on how the game works, both mechanically and meta wise. That's the sort of thing you need to get people to come and stay.
Their use of a traditional forum layout also surely helps, as it means it's much easier to, for instance, look up specific strategy advice, because strategy advice is all sorted into it's own sub forum. Or look for a replacement to a Civil Disorder, because again, that has it's own specific subforum. I know there's questions about how a new forum software would interface with the site, but it is something that would be needed to help bring new players in, even if it's something as simple as vbulletin.
Lastly, and I think this is less of an issue, but still a minor niggle, the ability to have a seperate adjuncation length for orders phases and build/retreat phases goes a long way, as it means you can enter a 2-3 day game, knowing the 2-3 days will be negotiation days about orders, while the game won't sit till Friday if Italy can't decide where to send its dislodged unit in Sweeden of all places.
Just my two cents from an outsider perspective.