I played with this on a members forum I built for kids. The problem that I ran into was two fold:

1) I ended up with an elite group of members, most of whom were very technical and knew how to work the system. New members didn't get no love. ;-)

2) Membership growth was terrible - I guess part of that was that we didn't have enough hooks to make the whole process of registering and posting for first introductory post (which is what made is possible to continue to participate - you started with nothing until you put up an intro post and got approved).

In some ways, registration is itself a form of gating. Putting a significant registration system on the frontend, which includes validation, provides enough blocking power to keep much of the spam off. But when it comes to building user communities, I have found (at least based on this one project - it was for 16-21 year olds) systems need to be friendly to newbies or they just don't get adopted.