HBP is actually up there with POA as my favorite books of the series; I purposely didn't reread it so I wouldn't focus on the things they would have to leave out, because they do have to leave these things out; the movie's 153 minutes as it is. I'm pretty easygoing about these things, so while I thought of some odd omissions in retrospect ("I'm the Half-Blood Prince" doesn't actually tell the audience what the hell a half-blood prince is), at the time, I thought it worked as a movie. I mean, yes, I would have liked a shitload of Tom Riddle memory-flashbacks, but I'd also like a million dollars and a pony. We can't have it all.
The one problem I really had was with two scenes that were included, because YOU DID IT WRONG tends to bother me more than YOU DIDN'T DO IT AT ALL.* And "wrong" may be overstating the case here. But in both these two scenes, we have exactly the same problem: a moment full of energy and chaos is slooooowed down. The first is the Harry-Ginny kiss, which I loved in the book because it was like they just jumped each other, which (for some reason) just always seemed right to me, and now in the movie it's this really weird, slow, ponderous, almost creepy thing. Like, seriously: the kid's had a hard life. Let Harry have a joyous snog once in a while. They did the same thing in the last movie with the Cho kiss, where it was all slooooow and momentous and whatever, but--well: Cho. I liked Ginny bolting through fire
* Exception: they left out Dumbledore's line about not being afraid because he was with Harry. This may have been for the best, because I would have bawled if they had left it in.
The other scene with the same problem is Snape and the Death Eaters leaving the school, and it's not so bad--I guess it works on its own, but I loved how helter-skelter the whole thing was in the book. (I'm totally okay with the Death Eaters not actually getting in a big fight with the kids and Aurors, because that's basically the second half of Deathly Hallows anyway--kind of redundant. But when you remove that fight... you don't really have any reason for the Death Eaters to be there. I mean, Draco and Snape pretty much have the Dumbledeath covered between the two of them. But if you don't have the Death Eaters sneak in, you don't have anything to do with the Vanishing Cabinets at all, so you pretty much have nothing for Harry to notice him being suspicious about. So you have to have them sneak in. But now that they don't have anything to do... they're basically just there for moral support. And I don't know about you, but when I think "moral support," I totally think "Bellatrix LeStrange.")
So anyway, no one really noticed that the Death Eaters were there, so they can blow away a couple of security wizards and stroll on out, that's fine, but I loved in the book how there was all the running and the taunting and Harry losing his shit and trying to throw curses at Snape and Snape being all like BITCH, SAY IT LIKE YOU MEAN IT, and in the movie, it's like, he knocks Harry over a couple of times while Bellatrix is toasting marshmallows around Hagrid's hut and then rolls on home to Voldemort. Oooookay. Maybe I'll like it better the second time I see it, I don't know.
And yeah, the movie is really, really funny--it's a nice mix of light and dark that keeps the movie from being too ponderous. Although you can totally tell that Kloves is back on script duty because suddenly there's multiple scenes about how Hermione is Harry's best friend omg and only they understand each other's lovelorn pain (although I did laugh SO HARD near the beginning when Dumbledore's all like, "So what's up with you and Granger? Icky like a sister, you say? Awesome, just wanted to twist the shipper knife a little before we get started with the actual plot") and Ron's all in the background like, "Remember that last movie, when I actually got to say things? Good times."
Anyway. I should probably save it for That Of Which We Do Not Speak. Cough.