Last time I did that we had about 12 forums on a particular site and needed to add a new one which was kind of unique/special (long story). We told everyone it was members only to keep quality high.
Everyone was very happy about that. Over a period of six months or so following, we slowly made all the other forums, one by one, members only to post, guests can still read.
We had no complaints. Membership in the forum (usernames basically) is a really important aspect to a community, so I think you have the right approach.
I think the most important thing is your registration policy - be very frank about why you want people to join, and re-assure them that the email addy they give is not going anywhere else (assuming that is the case).
Also, as mentioned above, do start interesting and detailed threads yourself to encourage people to get involved. There is nothing worse than an empty forum.
And don't go and create 20 forums all with wonderful descriptions. They will remain empty and look bad. Start small and let it grow organically. Only start a new forum if you can start a high quality and useful first post in it.