1. It's harder to find your community. Once you password or block content, it can't be used to attract members. It wont show up in search results. It wont be linked to by bloggers/websites. It wont be discussed outside of the community. You make it less likely existing members will pass it on to friends. These are big problems.
2. You're more likely to repel potential members. Registration is a time and information risk. You risk your time and your personal information in exchange for participating. If you can't show members what they're going to experience, why should they join.
3. Registration means nothing without participation. A member that registers just to read the content is as valuable (or as worthless) as the anonymous ip address who reads the discussions.
It's participation that matters. If you keep your content visible to all, members will only join when they want to participate. No sooner, no later. This is a good thing.