Best book in the series so far. Although, she does snip up some loose ends rather curtly.
Mark my words, don't write Snape off yet. Remember, Dumbledore KNEW that Snape was obligated to kill him. Especially in front of other Death Eaters. Also, the argument between Bellatrix and Snape at the beginning of the book becomes much more relevant. Don't forget, Snape aided The Order in more ways than Bellatrix knows.
Rowling said before the book was even released that neither Harry nor Lord Voldemort was the HBP.
Personally, I think you were blind/naive to not foresee the Ron/Hermione coupling. If you're over the age of about 16, the love/hate relationship they have is rather cliche. Harry/Ginny was a moderate surprise.
I am currently working under the assumption that R.A.B. MUST be Regulus. Regulus was killed by Volde himself, which was supposed to be a rare honor.
More entertaining than thinking about who R.A.B. is, is thinking about who helped him. Remember, Harry and Dumbledore had to work together to get the Horcrux, and Harry even says/thinks that it would take two people to get it. Regulus was dead at 18 and someone helped him get the Horcrux, who? I'm not willing to say Snape, but I could easily entertain the notion.
One last entertaining, mostly baseless thought; Voldemort's bond with his snake is much akin to Dumbledore's bond with Fawkes and what better Horcrux than a bird that won't die, the symbol of The Order, the one thing that Harry thinks he sees rising from Dumbledore's pyre?