I would like to add the max_enrolment patch (http://moodle.org/mod/data/view.php?d=13&rid=1241) to the core.
The patch allows a per course limit (including no limit) on the number of students allowed to register for a course.
Because it involves patches to important core Moodle files it tends to need maintenance and updating every week which is a bit of work. I would have to change the code if it was part of the core, as it should be completely silent if people do not want a limit on enrolments. Also the upgrade code would need to be changed.
I have added this here so people can vote on it.