Syd is a Death Eater! (Not that surprising, actually...)
I Knew It: Most people guessed beforehand who R.A.B. was, Sirius' brother Regulus Black, even correctly guessing his middle name Arcturus, from his grandfather on the Black Family Tree.
Pandering to the Base: Allegedly, the reason Umbridge was put in the book was this - to watch her get mocked even more.
Trolling Creator: Leading up the book's release, Rolling kept hinting that she may Torch the Franchise and Run by killing Harry so that no sequel books could be written about him. Which she did, but not permanently.