Just my two cents on the issue:
I can't recall why I chose webDip over other options when I first started playing diplomacy. However, I've always felt our maps, message and orders systems were very friendly and easy to understand (and eventually the issue with colours for coloublind people was solved. Not my case, but I appreciate the effort to welcome those who have it).
My girlfriend was at first startled that a map full of arrows of different colours could possibly be easy to understand, but ours is, if compared to others, quite easy to follow.
I recently joined vDip as well, where I've played two interesting games so far. For some unknown reason, though, I did not feel as much at ease there as I do here. Naturally, there are fewer games going on, and some of the features (such as picking your country) have relevant drawbacks (it also puzzled me why PPSC is so popular over there).
But certainly the most relevant element which ties me to webDip is the active forum (in spite of the change it has undergone some years ago). I seldom post anything, as I'm not particularly comfortable expressing myself publicly in English, but rarely do I fail to read the most interesting threads, from tactics to politics, sports to ethics, whatever. It somehow makes you get the feeling that there is a community. You come to, in a way, know people - their political stances, style of writing, sense of humour. However strange it might be, it strikes me as comforting to know people are here and talking, even if I'm for some reason not keeping up with the threads. It is as if that gave me some assurance that the site - and resulting community - is alive (though not necessarily, as pointed in the opening post, well).