The story behind Bladerunner is an incredibly interesting one. While I was away I brought with me a book called Future Noir which basically tracked Bladerunner from pre-production to about 1995 or so. Not many movies I know are a total financial failure in it's initial theatrical run, only to be re-released in theaters to sold out crowds and becoming a classic. One day we may be saying this about Gigli. One day....
The reason behind so many different versions are mainly centered around political trouble with studios and the growing popularity of the film. One example of this is the much maligned 'Narration' which originally came out. So much critical response basically forced it out in later versions [as the film became a cult hit] By the time the director's cut came out in 1992, there were actually two versions being edited, plus a couple of old sneak preview versions that changed. The two versions being edited for release in '92 was one that Scott wanted, and one the studios were making as a back up in case he didn't finish it on time. He didn't finish it on time, but the film released was somewhat of a halfway point between the two versions.
Anyways I'll try and list them here somewhat using the book [which incidentally devotes about 20 pages listing the difference of each version]
There are essentially 6 different versions of Bladerunner out there, albiet on so many different formats it must seem like there's more.
1) There is the Workprint version. This version was shown at Denver and Dallas as a Sneak preview/audience tester. This one has apparently 70 differences in it from the Directors Cut/International Cut etc. This is also the 'rawest' version as it was shown near the end of post-production and had things like different music cues, and differently edited sequences.
The workprint was thought to be lost until it was discovered unwittingly in the vaults of I think Warner Bros studios. It was thought to be a 70mm print [I think] of Bladerunner's theatrical release and was borrowed out to a film festival who were shocked to realise they were watching the lost 'workprint' version.
2) The San Diego Sneak Preview version. Shown once only in 1982. This had essentially three extra shots which weren't in the Workprint or any other version.
3) The Domestic Cut, original US theatrical release. The movie that was initially a flop, that became a classic later in life. This one had the narration and the 'happy ending'.
4) The International Cut, theatrical version shown in Europe and Asia This is probably one of the most common [next to the Directors cut I suppose]. This is a more violent version than the US, and is apparently more responsible for BR's cult rise.
5) The Director's Cut. Not really a Director's cut [since Scott couldn't finish the project on time.] Essentially the same as the domestic cut, except no narration at all, not the violence of the domestic cut and no happy ending, unlike the two theatrical versions. This version also has the 'mysterious' unicorn dream sequence. This is of course my favourite so far.
6) The Broadcast version
This is an edited version that was used for TV broadcast and eventaully poached for some, but not all of the video/laser disc releases. This has toned down violence and no swearing.
Yes, that's quite a few versions... I'm sure the versions released on dvd after the book was written conform somewhat to all of the above. Most of the time you just get different special features on different discs. I know the original BladeRunner Directors Cut didn't have pretty much anything in the way of special features...
This final cut will probably be much closer to what Scott wanted to release initially during that Director's cut run [the one he couldn't finish in time, a story in itself which is quite interesting]. I suspect that the main change here will be the addition of "Holden's hospital scene" in which Ford/Deckard visits his fellow Bladerunner in hospital as he recovers from being shot by Leon. There are probably a few other changes I'm sure, but that will be the big 'un.
If you're interested in the story of this movie's production, Future Noir is an excellent source of information. It's also a valuable tool for people in who want to work in the industry, because it tracks every stage of the production, from pre to post production, music, special effects and how they achieved them and so on. It certainly seemed like a mammoth struggle. It was a pain, because it made me want to watch the movie real bad whilst overseas and nowhere near a proper rental place!
I'm excited about this release since it looks damn cool [It comes with a model spinner I believe. God I'm a sucker for cool packaging, even if it's cheap.]
Hope this overview helped.