Elite committers vs. Gated trunk
How to control what gets into your open source project code ? The classic model, inherited from pre-DVCS days, is to have a set of "committers" that are trusted with direct access while the vast majority of project "contributors" must kindly ask them to sponsor their patches. You can find that model in a lot of projects, including most Linux distributions. This model doesn't scale that well -- even trusted individuals are error-prone, nobody should escape peer review. But the main issue is the binary nature of the committer power: it divides your community (us vs. them) and does not really encourage contribution.
The solution to this is to implement a gated trunk with a code review system like GitHub pull requests or Launchpad branch merge proposals. Your "committers" become "core developers" that have a casting vote on whether the proposal should be merged. Everyone goes through the peer review process, and the peer review process is open for everyone: your "contributors" become "developers" that can comment too. You reduce the risk of human error and the community is much healthier, but some issues remain: your core developers can still (wittingly or unwittingly) evade peer review, and the final merge process is human and error-prone.
The solution is to add more automation, and not trust humans with direct repository access anymore. An "automated gated trunk" bot can watch for reviews and when a set of pre-defined rules are met (human approvals, testsuites passed, etc.), trigger the trunk merge automatically. This removes human error from the process, and effectively turns your "core developers" into "reviewers". This last aspect makes for a very healthy development community: there is no elite group anymore, just a developer subgroup with additional review duties.
In OpenStack, we used Tarmac in conjunction with Launchpad/bzr code review in our first attempt to implement this. As we considered migration to git, the lack of support for tracking formal approvals in GitHub code review prevented the implementation of a complex automated gated trunk on top of GitHub, so we deployed Gerrit. I was a bit resisting the addition of a new tool to our toolset mix, but the incredible Monty Taylor and Jim Blair did a great integration job, and I realize now that this gives us a lot more flexibility and room for future evolution. For example I like that some tests can be run when the change is proposed, rather than only after the change is approved (which results in superfluous review roundtrips).
At the end of the day, gated trunk automation helps in having a welcoming, non-elitist (and lazy) developer community. I wish more projects, especially distributions, would adopt it.