there's quite a few things that have been cut or added through the entire film, the "nest" scene is the single biggest addition though.
There's been some fairly significant colour timing/correction/grading been done to it too, so visually it looks quite different to the original version (if you've seen it as many times as I have you'll notice straight away)
Ridley Scott maintains it's not a "directors cut" though, just an alternate version. Personally I prefer the original theatrical version, none of the new material really adds anything and the use of the nest scene makes a mockery of all the films that followed it in the series.
The new audio mixes are nice

and the extras are excellent it's just the new version of the film wasn't really needed, but you have the option to watch either version so you can't lose
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