I saw the matinee of Heisenberg today as well and enjoyed it just as much as I did when I saw it last year at Stage II. To address the question of what has changed, briefly: nothing. One of my biggest fears about the transfer is that the original production would lose the intimacy of the space that I found so integral to the success of the show. I had a great on-stage seat for the show today, so I can't describe how this plays up to the mezzanine, but I think the idea of taking 200 seats out of the mezzanine and putting them on the stage was brilliant and tremendously helps the show feel like it fits in the Friedman.
So much has been said about Mary-Louise Parker's performance already, and she deserves all of the praise she has and will receive. Her character propels the play forward, and she brings so much humor and heart to a role that could, in less capable hands, be played merely as an eccentric woman. I, too, hope she's remembered in the Spring with a Tony nomination.
I do feel like not enough has been said about Denis Arndt, though, who would also be a deserving Tony nominee for his understated, yet tremendously resonant performance. He certainly has a more passive and quiet role in the proceedings, especially at the beginning of the show, but he works so well as a complement to Parker and he is so warm and endearing. His brief moment alone on the stage during the mid-point of the show is one of the more emotional portions of the show, too.
Although I had hoped for a bit more elaborate staging for its Broadway transfer, I'm ultimately happy that they kept this production as bare-bones as the original Off-Broadway because, as bwayphreak234 said, it helps focus your attention on just the relationship between the two characters and the two actors. As for the play itself, I could understand how one might think it's "slight," but I think Simon Stephens has written a beautiful, simple, and elegant two-hander that's thought-provoking without being too heavy or depressing. Speaking of Stephens, he was in the audience today, and on our way out of the theater, we ended up walking next to each other in the aisle. I complimented him on the work, and we had a nice conversation about the play and the transfer from Stage II to the Friedman; he was truly lovely and appreciative of the support.
I wouldn't be surprised if this ends up being one of my top ten shows of this season. Highly recommended!