Is this strictly for techies that would possess AWS keys in the first place (see Dropbox HN launch post, just set up your own FTP server, etc) or eventually intended for normal people?

If you are implementing client side encryption, something like "log in with dropbox" oauth, GDrive, etc or similar to just serialize an encrypted snapshot into a known folder would be the most usable for non techies based on previous stuff I've worked on. Asking people to set up restricted IAM roles and keys to write to a single bucket for a todo list is honestly very high up there on the ridiculous list, as much as I would totally do that.