You hit the nail on the head about Langella and Nixon's loneliness and incredible arrogance.
I've always been fascinated by Nixon, and this just adds to it. He was forced to do so much in spite of his ideals and political beliefs, but still managed to overshadow his political life by such obvious and stupid mistakes.
I've seen many movies recently that were originally plays, such as Closer. One can see HOW this could be staged as a play, but the movie never feels confined as Closer did, trying too hard to relegate the action to static locations, or even confine the action to just a few key characters. It all feels quite natural.
I also was surprised at how smart the whole thing was. Nixon's jokes usually take historical knowledge (that should be common, but I fear is anything but). There is no explanation about Russian leaders always being taped: it's just funny. I appreciate that, especially in a Ron Howard movie. I felt smart after seeing it!