Hi - the sign up form requires a name and URL of your blog and the date it began - this may be discouraging for those who don't have blogs yet. Can you make that information voluntary instead of mandatory?
I have found ning's CSS to be almost irrational. I've tried to customize pages here to no avail: 256 "web-safe" colors (my god! there has to be at *least* 65,000 today!), garish themes, and virtually no semantics to help. Good luck! (glad I'm not doing it!)