1. Quit the app(s) you want to configure.
2. Launch Terminal.
3. Paste the command below in the Terminal window, then replace 'name of app' with the name of whatever app you want to configure (e.g. 'textedit'), before hitting Enter:
defaults write -app 'name of app' ApplePersistence -bool no
Alternatively, you can configure all apps at once with the following command:
defaults write -g ApplePersistence -bool no
I don't recommend doing this, though, because, for some reason, it slows down the login process considerably.
For TextEdit only, you also need to run another command, as a workaround for errors that prevents it from functioning correctly:
defaults write -app textedit AutosavingDelay -int 0
4. Relaunch the app(s).
5. Enjoy you considerably less infuriating Mac!
If you don't see any changes, a relogin will in most cases do the trick (if not, the app probably uses a non-standard saving mechanism).
Please note: Although this brings back Save As in most apps, this is sadly not the case with Preview and iWork.
To undo any of the above commands, run:
defaults delete -app 'name of app' ApplePersistence defaults delete -g ApplePersistence defaults delete -app textedit AutosavingDelay
P.S. If you want to remove data already stored by Versions, you need to delete the hidden .DocumentRevisions-V100 folder in the root directory of your disk(s).
Because the "Ask to keep changes when closing documents" setting doesn't actually turn off Auto Save and can cause loss of data.
Because the Save As "feature" in Mountain Lion, isn't actually the traditional Save As that OS X had prior to Lion, but a useless, mock version that is likely to cause data loss.
Because Apple still refuses to offer any way of disabling Versions.
Or, in other words: