The play is actually much more about Frost; Nixon is merely a slide player, a vehicle for his return for fame and fortune in New York. And what it shows more than anything is the large part that luck plays in all these sorts of high-wire resurrection acts: if Jim Reston hadn't found that document...
But mostly, the most interesting and revelatory stuff is contained in asides, and are subtly highlighted through Michael Grandage's direction. The one that lingered most was Nixon's walk off stage after his victory salute into the helicopter: he still clung to the belief that he could come back, and he wasn't ready to leave the stage yet.
Similarly, Nixon's rambling, drunken and, most-likely imagined, late-night phone call to Frost in his hotel room, is brilliantly revealing of the strains of ambition, especially for 'jumped-up country boys', like the two of them were.
What with the adaptation of The Last King of Scotland and The Other Boleyn Girl forthcoming, and The Queen packing them in, this is clearly Peter Morgan's imperial moment. And Frost/Nixon is a very purple patch indeed.