I just added a new FAQ item for that, but I'll copy it here as well:

We plan to reduce low-quality contributions through several features, some already in place and others still to come. Invite-only registration helps us discourage low-quality accounts and confirm that new members are real people. Next, we plan to introduce a vouching system, similar to Tangled's, that lets contributors build a reputation. We'll combine that reputation with other signals to assess new pull requests and issues. Contributions that appear low quality will be clearly marked and de-emphasized, while maintainers will make the final decision. We also plan to limit how many pull requests and issues a new contributor can open in repositories they don't own. This will be an ongoing effort, but the goal is simple: maintainers stay in control of their queues.