Your Keynote experience, Stefano, is precisely the kind of problem Apple needs to recognize. Keynote is, as elol added, one of the applications Apple is most fond of. I love Keynote, and I can't imagine using anything else to create a presentation.
Some of my Keynote files and small, text-only, and intended to be exported to Power Point to share with others less fortunate than us. In fact, the now-discontinued iWork.com beta site used to be outstanding for giving viewers of the file the option to download the format of their choice, or download nothing at all and just view it. But I digress ...
Other Keynote files are larger because they include many photos. Often, people don't realize that the photos they insert into a Keynote file need not be high resolution when the presentation is not being printed, but only viewed on a display device. Nevertheless, it doesn't take many photos or Quick Time movies to make a Keynote file quite large. Add a soundtrack with music or narration and it gets even bigger. And we don't need to waste valuable disk space storing so many incremental copies of the file.
Moreover, if you were to use the Revert command and needed to decide which verion to go back to --- how would you differentiate one from the other. Maybe the slides themselves provide a clue - but maybe the changes you made involved builds or transitions from one slide to another, and you could waste hours examining the earlier editions trying to find the last one that preceded the "error" you made that caused you to desire to revert in the first place.
And, to add insult to injury, while you're taking all that time exploring your previous versions of the file, Auto-Save is still working away, making new versions of your current file.
Conclusion: Keynote should definitely allow users to specify if they want auto-saving turned on or off, perhaps with a user-specified time interval. For someone who wants to use SAVE AS instead, that option should also be there.
Given that Keynote also works on the iOS platform, where storage space is more limited than on desktop computers, the auto-saving makes more sense, it is worth noting that there is no REVERT command. Search the Keynote HELP file for "revert" and you won't find anything. If you edit a slide and find the need to go backward, you get an UNDO button. No saving, no reverting. Does it make sense on the mobile device? As far as I'm concerned, it's good enough for me.
But on the desktop, which I still consider my "real" work environment, I want more control.