~~I think it comes down to why someone is training. There have been many good points brought up here, but few dojo unfortunatly can cover them all. My dojo has a definate 'in group' where I have never really felt welcome, especially in my beginning years, so if the social connection was quite important to me I'd have left long ago. I'm there for the level of training and hang with myself or the few friends I have made and that's fine...now.
~~A dojo organization needs to try to meet as many of those pivital points as necessary, but will never meet them all. Perhaps the social chemistry is great but the chief instructor is only so-so; maybe lots of classes offered but training is always hard and so turns off most women, etc.
1) Good instructor(s)
2) Solid curiccumlum
3) Consistancy in the program
4) Genuine welcome/interest, not something to simply 'get them in the door'
5) A true sense of community, not just for some but for all
I believe a dojo needs as many of these points possible not only for beginners but to keep the regulars, as Peter pointed out. If too many points are lacking, no matter what kept them going for many years, the regulars will fall by the wayside as well. If enough points are present than the dojo will retain more beginners...and that's where long term regulars come from.