I would like to hear everyone's thoughts here too! I have got my WordPress basically down, but I just have a couple more things to do before I want people to sign up!
One thing that I am currently doing now before people sign-up is make sure there is a good selection of Themes, and be sure that there are a couple Plugins available. AKA Spam protection, etc...
And be sure that you are comfortable with your design on the home page to "attract" new users. I have a pretty small niche, but when I open up my wordpress mu to the public, it will take on really fast because it is something none of my users have seen before.
Be sure that your site is running PERFECTLY, and make a couple test blogs, etc...I would also recommend writing a page about how to "get started" for your users. Sometimes it is hard for people to start their own blog.
Hope this helps!
Christian