One way to reduce the cost of entry is to not make people create yet another username/password pair that they're going to have to remember (and that you're going to have to manage and keep secure). Instead, try using OpenID or OAuth, which let users use a credential they already have to sign into your site. Common credentials that can be used this way include Google, Facebook, Yahoo, and assorted blogging sites. Combined with minimum requirements at your end, you can have new users up and running in seconds. Really, do you need them to provide anything more than an OpenID and some sort of user name? If you want them to fill out more information in a profile, you could defer that until later, once they know they're going to stick around.
It appears that phpBB doesn't support OpenID natively, but
this package supports OpenID for phpBB3. PhpBB 3.1 supports OAuth as an authentication provider.