I just noticed that TV Tropes has a
very good article on the subject of dieselpunk. They also break up the definition of what dieselpunk is into four categories, instead of our two (Ottensian and Piecraftian):
Diesel Deco
This is the most optimistic form of Diesel Punk. Progress seems unstoppable and the future is bright. Things are designed to be stylish and opulent, ornamental and efficient at the same time. Think Bauhaus architecture and design, Art Deco, Expressionism, the 1939 New York World Fair. A good setting for a Science Hero.
Diesel Noir
Similar to Diesel Deco, but generally Darker And Edgier. Emphasizes the downside of economic and technological progress. Society is plagued by crime and corruption, technology seems to be at its most effective in producing increasingly effective weaponry. The occult basically amounts to Black Magic (including exotic religions), Sealed Evil In A Can might turn up in an archeological dig and subsequently have to be stopped to avoid The End Of The World As We Know It.
Diesel Dystopia
World War II did start and may still be in progress; if it isn't, either some kind of Cold War is being waged, or a One World Order has been established. Either way, The Government is intrusive and ruthless, ostensibly to protect the citizens. The dictator might be Socialist, Fascist, or some other kind of totalitarian.
Diesel Desolation
World War II did start and ended because there isn't anything left to fight over, and very few resources left to fight with, or even to sustain civilization. It's essentially a post-apocalyptic milieu, and certainly not a very common Diesel Punk flavor.