I saw the director's cut yesterday afternoon. I'd never seen the original. I saw it because The Stranger (Seattle weekly freebie) gave it a good review, and because I recognized Jake Gyllenhall's name from having seen The Day After Tomorrow last week.
I really liked it, and the best thing was JG's performance. No matter how bombastic the movie got, he stayed believable.
I did some Google-ing about the film last night, and found a couple of links that may explain some things:
Donny Darko Theories and Explanations
http://www.tonystuff.co.uk/darko-spoilers.htm
Includes a link to a page with all the text from Roberta Sparrow's time travel book, which was on the DVD, but not in the original release. It has been added to the Director's Cut.
Barbelith Underground
http://www.barbelith.com/topic/3817/from/105
Scroll down to Chubby P's post to see a list of purported 'cheats' to read the background info on writer/director Richard Kelly's official website,
www.donniedarko.com. I haven't tried them out. I couldn't get past the third frame when I tried just pointing and clicking like a normal site. Maybe it works better with IE? (I'm using Safari).
According to the pages from Roberta Sparrow's book, the Living Receiver of the Artifact (which would be Donnie) is granted special powers in the Tangent Universe that he needs to fix the rift . Frank and Gretchen are able to act in both universes, and in the future, because they both died in the Tangent Universe. Or something like that.