So instead of "save" which seemed rather simple to me, you have to learn two new features such as "lock" and "revert", which do not seem so simple to me.
Oh and also, now you have no idea what data is saved in your documents. Your documents can carry in them hidden past versions without you being aware of it. Which can prove very embarrassing in many business situations. Which means that anyone that handles sensitive data will have to create or buy complex new software that scrubs all documents of old data. (this is already an issue with Microsoft Word and its proclivity to save all kinds of dangerous metadata).
So when before you merely had to save stuff, now you have to worry about reverting, scrubbing, etc. Thank you GUI elves.
The thing is, you tend to hit save every other second, while you probably only need to revert/lock like once or twice a day.
Think of lock just like commiting to your personal source control. Revert than maps perfectly to SVN revert. You work just like you always do. You commit whenever you finished some task and want to start a new one. You revert whenever you found out that you did some wrong stuff since your last commit. I do these actions every day today.
But I almost never discard all changes since the last save. Hence, I'd be glad to have the OS save my work implicitly for me and take care of my personal source control system. The proposed system would actually save me a lot of work!
I think that losing unsaved work is a bigger and more common problem than those downsides you mention. Locking and reverting is not something mandatory, users can simply ignore those features until they need them. On the whole I see automatic saving as a nice step towards computers as an appliance that can be used with less and less regard to implementation details. (The security side is an interesting point, I wonder how they solve it.)
Oh and also, now you have no idea what data is saved in your documents. Your documents can carry in them hidden past versions without you being aware of it. Which can prove very embarrassing in many business situations. Which means that anyone that handles sensitive data will have to create or buy complex new software that scrubs all documents of old data. (this is already an issue with Microsoft Word and its proclivity to save all kinds of dangerous metadata).
So when before you merely had to save stuff, now you have to worry about reverting, scrubbing, etc. Thank you GUI elves.