The things that were good:
Acting: Exceptional. Especially Eddie Redmayne, recently in Tess of the D'Urbevilles, and sporting an impressively whiny American accent.
Themes: More complex than I anticipated. And handled well. I didn't know what I thought by the end, and this was a good thing.
Overall niceness: I don't know how to describe this, but there was a real warmth to the play, and some good humour.
The bits that weren't so good? Well, it was a tad didactic at times, feeling the need to explain US political history in a rather clunky way. And there were a few moments when it went a bit Dawson's Creek Election Special. You know, everyone expressing themselves very eloquently, and showing off their full range of vocabulary and tortured vowels, and doing those unnaturally long monologues while other actors ostentatiously nod at appropriate moments.
But these are small things. Still an excellent play. And now I know how hard it is to even try to write a play, I'm all the more impressed by people who actually get things written, and written well.