To be honest, I think the whole idea of revolving films around dreams is quite overdone and easy to do because it is so subjective as to what things "really mean." Nonetheless, I thought the script was pretty unique and had some very strong aspects to it.
The dialogue seemed pretty realistic and well written for the most part (especially for Kurt's character), but when Nolan talks with Professor Walker (scene 13), the conversation seems a little forced and unnatural, as if you're just trying to squeeze out reason. Having written many short scripts myself, I know how difficult it is to try to get a point across in a concise and short way; imho, you should think about extending that scene just a little - just doesn't seem like a realistic conversation with a professor and a student.
I liked the ending, and how you summed up the concept of the film with the envelope, but I don't like how it was foreshadowed at the beginning in the way so many student films (and unfortunately some independent and even professional films are) where "the beginning is the end, and the end is the beginning" type sh*t. I would alter it slightly and make it not so obvious that it really is the end (because I thought to myself, "this is probably a foreshadow of the ending), but in fact, just a frightening dream of him falling (maybe a POV shot of him falling to the ground and then taking the deep breath) - just a suggestion.
In terms of characters, I thought you developed them very naturally. I hate it when stuff is blatantly explained so the audience can "understand" what kind of person they are, and you did a great job of avoiding that and breifly incorporating subtle characteristics about him that made (me at least) very curious as to who this guy actually is and what his problems are. The only problem I had was the sex scene, was it really necessary? We already understand that they are going out, and after finishing the script, I asked myself "what the hell was the point of that scene?" But that's just my opinion, maybe there's some psychological insight about Nolan and Danielle I'm missing, just seemed very arbitrary and pointless in advancing your story. I would have omitted it.
Overall I thought this script was pretty good despite my feelings toward "dream-films." I would really like to see how this project turns out. I hope some of my advice helped. Good luck.