I'm with everyone on Dumbledore and Snape. Very unexpected and quite unbelievable. I was aware of the rumours, hoping against hope it wouldn't be true. It just doesn't make sense to me, for all the reasons already mentioned.
I had a high opinion of Dumbledore's judgement and find it very hard to believe he made a huge mistake with Snape. I'm sure there's more to it. After all, how many times did Harry raise his suspicions about Draco and Snape? And time and again did DD say Harry was wrong, while all along DD knew about Draco's intentions. So he should have known about Snape. Especially since he knew how good Snape was at Occlumency. There's definitely more to all this.
I do find Harry doesn't deal with panick very well. At first in the train, then on the island with DD and finally on the tower. Harry had a chance to save Dumbledore... he could have used a non-verbal charm like he tried in the train. I am quite surprised Rowling didn't look into that, or in more of Harry's deeper feelings. I feel there's more to explore there (yeah, I know that's the angst area, what can I say <g>).
In any case, I look forward to the last book. I'm sure we'll all be surprised at the outcome there.
Saskia
