Dan wrote that he and Blair had sex, which she insists they did not but almost ruined her engagement anyway, as she told Louis earlier (before reading the book) that he had likely written the truth.
Dan wrote that Serena was a selfish, narcissistic party animal, which apparently cost her boss Daniel-Day Lewis in a major movie, but her boss is dead set on getting the Inside movie rights now.
He made Chuck kill himself and die alone, which makes the Bass man awfully introspective about where his life has been and where it's going from here.
He made Nate gay, which he was cool with, but he also blended Nate and Eric into one basically irrelevant character, which makes Nate sad/jealous.
Basically everyone hates Dan now.
Meanwhile, Diana learns incriminating information about one of Nate's "friends" - IVY - and uses it to her advantage. In return for not outing who she is, Diana makes Ivy work for her.
(Source: TV Fanatic)