He didn't go back to the front line, so his work had more distance and perspective and, Battle of Britain aside, it was less about the heart and more about the head. Another major difference was that his brief was more to do with propaganda and less to do with pure, visceral art. A third was that during the 1930's Surrealism had finally crossed the Channel and started to influence English artists, none more so than Nash.
I've always had more than my fair share of issues with Surrealism. I appreciate the impulse but too much of it (cough - Dali!) appears forced, arbitrary and formulaic when it should be spectacular, emotional and arresting (like Ernst at his best). Nash himself had a curious relationship with the movement, falling onto both sides of that divide at different times and eventually declaring himself to be "for, but not with" surrealism. His work had always had a tendency to see magical transformations in the landscape, to make unexpected juxtapositions and to draw similes quite literally. I wrote an essay many years ago which I'll rework and post here; I find Nash's links with Surrealism fascinating but don't want to get too sidetracked ;-)
Anyway, long story short, sometimes Nash + Surrealism + WW2 = startling image, and here is one of the lesser known ones: Follow the Fuhrer Above the Clouds. It was rejected as propaganda and is now hidden away in the archives of the Imperial War Museum.