I think paid accounts could be used as a document backup service as you get access to unlimited history but with free accounts I believe backups are only stored for 30 days.

The important bit is "one where I don't have to set _anything_"—you do have to put the files you want synchronized into your Dropbox folder. (This is the part I've found hard to explain to people that just save things to wherever the Save dialog opens.)

This could actually be worked around; once Dropbox enables per-folder sync, they could also provide a daemon (as an option) that automatically finds and sets up sync for any folder you seem to be making personal use of. Kind of like how Picasa works with pictures, but with everything on your computer that's yours.