I suppose it's equally as likely. The example I'm thinking of is losing your network connection while typing an e-mail in GMail. It shows a warning message, but out of the way so you might not notice. If you intentionally take an action to save, you are more likely to notice the result (and a good user interface would make a failed intentional save obvious by a popup message or something).

You made a change - that's it. You don't save your paper after drawing on it with a pencil, do you?

Depends. If I was drawing something remotely important, there would probably be many revisions, some of which I'd want to keep, some of which I'd want to throw away. Some of which I'd want to make a separate copy of to work in a different direction. So yeah, versioning. How do you version things without files?

At the very least you have iterations represented as dates, fragmenting whatever metaphor is supposed to replace the filesystem.

Tags have the same problem, once the list of tagged items becomes too big tags get used as directories. Tagging seems best at showing popularity/frequency, but this does not replace directories in any meaningful way.