To save loads of database space, some sort of plugin that would purge unused / inactive blogs would be very, very helpful. Ideally it would:
Send a notification email to blogs xx days before deletion.
Mark blog as inactive if not posted on in xx days (so that you can recover in event of reclaim).
Purge blogs if never posted on (other than default post) after xx days.
Purge blogs that haven't been updated in xx days.
I'm sure most hosts have at least half their databases filled with unused blogs, this would be a great resource saver and much easier than searching / deleting manually from wp-admin.