It seems a little underhanded, but try and populate your forums with a decent number of posts before you launch. Either that, or don't launch your forums until you have a decent number of users. An empty forum has a pretty significant impact on someone who is casually looking at your site. Whether that reaction is justified or not, you still have to recognize it. You might consider a mailing list as a temporary alternative.

Also, I'm not sure how essential the user's email is to the sign up process, but if you can take that part out, I would go ahead and do it. That will encourage people to register, and you can give them an incentive to add their email later. For example, on the admin page you could tell them to enter in their email in order to receive notifications about the status of their backup, etc. That's mostly just a personal thing for me. It's not like your breaking any rules by using an email in the sign up process. In fact, it's nice not to have to deal with remembering a user name.

More advice: when you launch send a press kit complete with a company summery, personal bio, relevant media (images, video, etc), and a free registration key to all the popular tech blogs. People are inherently lazy, and if you hand an author all the tools needed to quickly write an article, plus a free premium version of your product, it's likely you can generate some good press. It also might be a good idea to include the press kit on your site for easy downloading.