I Knew It: Most people guessed beforehand who R.A.B. was, Sirius' brother Regulus Black, with some even correctly guessing his middle name Arcturus, from his grandfather on the Black Family Tree.
Notably, Emerson (from MuggleNet) and Melissa (from The Leaky Cauldron) were given the opportunity to interview Rowling the day after the book was released, immediately after their first read through. They both guessed R.A.B.'s identity immediately.
Pandering to the Base: Allegedly, the reason Umbridge was put in the book was this - to watch her get mocked even more.
Playing Gertrude: Daniel Radcliff, Emma Watson and Rupert Grint in the epilogue.
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.
What Could Have Been: A lot of material released after the book's release (such as the trio's lives after Hogwarts) were trivia Rowling would've liked to include in the actual book.
A deleted scene for Deathly Hallows: Part II would've shown Hagrid fighting the Death Eaters and ultimately being captured by them; as this was removed, Hagrid's first appearance is simply him already captured.
There was serious consideration for filming the deaths of Lupin and Tonks.
Working titles for the book were Harry Potter and the Elder Wand and Harry Potter and the Peverell Quest. The latter was rejected for being way too cheesy, but the former would have made a lot more sense, especially since the other Hallows (the Resurrection Stone and the Invisibility Cloak) aren't really important to the story the way the Wand is, except in the thematic sense. The Stone is only discovered towards the end and put to use exactly once, and while the Cloak sees a fair amount of use, it's nothing that Harry hasn't done before, and it isn't over-used. Though it must be noted that Harry only uses the Elder Wand once, to repair his original holly-phoenix feather one.