It would be nice to be able to put Bamboo into maintenance mode. After turning this on, it would remain in maintenance mode until either it was restarted or maintenance mode was disabled.
Maintenance mode would mean that builds would be allowed to finish, builds would be allowed to be queued, but no builds will be started. When maintenance mode ends, all queued builds will begin running (as agents become available). Queued builds should be persisted across restarts/upgrades.
The business case for this is to allow Bamboo upgrades without having to stop builds and make people cranky.