I like the ability to specify how many shows to keep, but sometimes you decide that you want to save a particular show before it gets auto-deleted. Either permanently, or just so you have time to watch it.
One of my cable company DVRs had a similar auto-delete function, but also had a "Save Until I Delete" option for each program so you could make sure it wouldn't get auto-deleted before you had a chance to watch it.
It would be great if this could be added to Channels.