I went pro like 2 months ago my site actually gets high traffic. Even I went throught the free site phase, heck my site was free till I laid the money down for all the cool stuff.
Having a premium account usually means ad-free. If you're are serious about an ad-free website you have to find an ISP that offers premium (paid subscription) accounts.
Having the registration required feature on certainly doesn't help since the visitors have no reason in taking time to register if the forum is calm. You have to enable posting for guests and cope with spam until you get at least 100 regular members but it might take at least a year unless you have some unique features. In other words nothing is as easy as it seems.