Something I found myself unprepared for is the bad-decision-as-an-assertion-of-power pattern. It literally was not until age 27 that I realized (despite seeing this behavior several times first-hand) how common it is, in business, for people in charge to make intentionally bad decisions for the sake of (a) asserting power, and (b) testing others' loyalty. I guess one could say I was too naive and hyper-rational to believe such a thing to be possible-- except in the movies.
I wish I had a solution. For the record, people who say "Startups!" don't know what they're talking about. Startups can be great, but office politics can get just as ugly in small companies.