How can I configure custom repeat intervals for Reminders?
This bugs me too. It isn't possible to do in Reminders, but you can achieve the same thing by creating a repeating event in Calendar, which has a "custom..." item in the Repeat menu:



Then you can specify exactly when to repeat the event:



It seems like an oversight that Reminders doesn't have the same feature (it would be just as useful), especially since the event/reminder detail UI is the same...