Stefan, you may want to also consider having a rule added for low priority items that change drastically get promoted to the "next cycle" of backup. Maybe have tiers of frequency. Like Oskarrough said, "Many folders I only need to backup once a week or even month", but if that folder change by 50% or more in a day, maybe that's something that needs to be updated sooner than the next month. Maybe have all backups setup with a medium priority, then the user can go in and "promote" or "demote" certain folders, etc. That way really important (to the individual user) data gets' the most updates to S3 and the lesser, nice-to-have data, is done less periodically. And, heck, add a button titled "EVERYTHING NOW" to grab all changes to the configured folders uploaded to S3 ASAP.
I know that may be aiming a little high for functionality, but that would be one kickin' feature.