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I though the real give away was how in the keynote, the guy gave the example of accidently saving instead of Save As, and that you could go back in time and undo that.

Whole lot of other previous solutions had this feature, it exists even on Windows Server. Some do log changes and some log file versions.

If Time Machine were designed to backup just once every midnight, you could hardly go back in time to undo specfic short term mistakes like that.

And you never question why previous versions never replaced scheduled backup? Scheduled backup still rules. I went for a few solutions like Time Machine so far and I always returned for the same reason, I solved one problem but got 20 new ones.

Let me give you a clue. You work on a large movie or picture. Simply change background and complete file is different (which will make changes even larger than original file). Now how many copies of that large file will exist?

This approach is acceptable for non-binary controllable formats, for binary (especially losy file types)? Not really. And the main trouble is that binary are usualy larger ones. Thay are maybe even acceptable for databases as they don't change whole file at once, but the next point about slowdown could prove it wrong (or not, because when db is written no new handle is created and changes could be intercepted and logged if this thing was implemented correctly).

Second problem here is how file is being written. Write to temporary file. Read old and new one to create a diff. Store diff and diff information to its internal structure. Rename temporary and delete previous.
Now imagine this write with 1GB file or even larger.