This suggestion would come at the cost of increasing the overall game (executable) file size that is stored on the HD, but it would address a recurring concern that keeps rearing it's ugly head through every update patch.
The basic premise is to have 2 copies of Minecraft stored on the same Hard Drive, each a a different Update version. what is considered to the be the 'Current Update', and what is considered to the the 'Next Update'.
This allows for when a new Content update rolls out past Certification Testing, the player has the option (after updating) at run time to continue playing under the Current (or previous) full version, or to play the game in Beta-Mode for testing purposes. (Beta Run game files would not be backwards compatible, but could preserve a previous version backup at the point of conversion to revert back to should the player change their mind and want to go back to playing in the same world under the previous version).
Bug Fix Updates would only apply to the Beta Side until 4J was confident that they have a viable production worth Bug fix for that Update prior to the next Content Update.
Alternatively, instead of calling it a Beta Testing Mode, the game could optionally just keep a copy of the previous Game Generation files until such time as the player wanted to release them.
This methodology helps to assuage some of the biggest complaints that I keep seeing crop up time and again: "I refuse to update my version of the game until it has run for awhile and I am satisfied that I can live with the bugs reported." : "This Update is unplayable/horrible/terrible/sucks/etc. please revert it back to the previous version or fix it immediately." : "Warning to all other players: Don't Update your game!!!"
This would allow players who want to stay current and try it out and test the newer content and provide constructive feedback to help improve the game the ability to continue to do so, while at the same time getting rid of the excuses to not update and whining about when they do update and things didn't quite go as expected.
Support. I have always believed that there should be a way to step back from an update if one discovers something (regardless of whether it is bug or not) that they don't like about it. Minecraft is a game and, as such, people should expect to be able to just enjoy their own game. (They purchased it after all). As it is on the Xbox, people who decide they don't like an update (for any reason) really have no great option to get them back to the game they were enjoying just prior to accepting an update. This makes for an ever growing number of unsatisfied customers who feel that their worlds have been "sabotaged" by the update process.. as opposed to just a group of people who might just be happy enough to step back after trying out an update to just play offline and stay out of date.
I've been told that one way around this issue is to just manually copy the existing update to a USB prior to accepting a new one; and then deleting the new one and re-copying the prior update back to the hard drive. Frankly, I've not tried this myself, so I really can't say whether or not it actually works. Still, since most people are usually so excited to get an update that they don't even take the time to back up their existing worlds manually, it would be nice if the game just didn't overwrite the previous update and that world saves associated with it and would automatically reactivate it and those world saves if the subsequent update was deleted. You could call it an update trial or demo version.
Requiring people to then physically "release" their old world saves in order to commit them to a new update is also, I think, a good idea.
As for the "cost" in hard drive space... well, Microsoft should have faced the fact by now that Minecraft is the sort of game that takes up, perhaps, more than its fair share of space in order to run well. A bit more space in order to change how it functions from an annoyance to the customer to something that is really convenient to the customer should be a "cost" they should be willing to at least try to work with.
Alternatively, they could just make all the older updates (or at least "key" older updates) available for ongoing manual redownload through Live.