For Clojure, there are several contributors with the authorization to be Clojure screeners (the list is here). They examine Clojure tickets and patches, deciding what to do with them. If they approve of a ticket with a patch, then Rich Hickey also examines the patch. If he approves, then the patch is committed.
The project leads, Clojure screeners, and Rich Hickey all work on Clojure as volunteers, in their spare time, many of them since 2007 when Clojure was first released.
Other articles that may be of interest:
Signing the Contributor Agreement grants Rich Hickey joint ownership of your contributions. In exchange, Rich Hickey guarantees that Clojure will always be available under an open-source license approved by either the Free Software Foundation or the Open Source Initiative.
Why does Clojure require that contributors submit a contributor agreement on paper?
The requirement to mail a signed CA on paper was set up in 2007 (TBD: verify if that is the correct year), before there were as many choices for electronic methods as exist today. It is the most easy to defend if there were ever a legal challenge to the copyright of any of the Clojure code. There has been some discussion in October 2012 on the Clojure Dev Google group about allowing electronic submission of CAs, too, but there is no process in place for that yet.
Link to Clojure Dev discussion thread
Other projects hosted on GitHub accept pull requests. Why not Clojure?
Rich Hickey prefers to evaluate patches attached to JIRA tickets. This is not to make it more difficult for contributors, or for legal reasons, but because of his work flow preferences. It is easier for himworkflow preferences. The process of creating JIRA tickets and patches is documented and not terribly difficult.
Link to Oct 2012 Clojure Google group message from Rich Hickey on this topic
But what about all of the people who won't contribute if you don't change your process to suit them?
If some people choose not to go through the steps for the current process, that is unfortunate. The process of creating JIRA tickets and patches is documented and not terribly difficult. We understand that submitting a physical signed CA is slow and/or expensive from many parts of the world, and hopefully there will be an electronic method in place in the future.