I wasn't really able to get out everything I wanted to say about The Half-Blood Prince in that capsule review. But I also didn't have time today to craft a proper review. So you're getting all my spillover thoughts in a list format. The laziest format there is!
A Bit More Explanation Would Have Been Nice: I'm coming at the movie having read the book, but even I thought that only giving cursory mention to Harry's mysterious textbook, only to have that "I'M the Half-Blood Prince" reveal as a near throwaway at the end was kind of laughable. I already mentioned how a bit more time spent explaining just what the fuck a horcrux is would have been nice (though, with two movies worth of time to present the final book, I guess there will be time for Horcrux 101, with flowcharts and stuff). And the Katie Bell stuff is a hell of a lot scarier and more immediate if we're given a little bit of context for her -- she was a friend of Harry's, they played Quiddich together, et cetera.
And Speaking of Things That Had Much More Gravity in the Book: We really needed to be able to see the damage Harry did to Draco with that sectum-sempra spell he used. I'm pretty sure the book mentioned huge, bloody gashes on, like, his face. It really took the reader aback and let you share in the revulsion Harry felt over what he was suddenly capable of. In the movie, it was too sanitized. There, those are all my nitpicks.
The Casting Agents Deserve Several Medals: I realize they pretty much had the entire United Kingdom at their beck and call, but more than any other movie in the series thus far, Half-Blood Prince is a triumph of casting actors who are able to make an impression in an instant. Mostly because that's often what they get. Alan Rickman has by now settled into having about three scenes per film, but he gives you everything you need on Snape in every one. Helena Bonham-Carter is having a fucking ball playing Bellatrix Lestrange, and she's actually done a better job of presenting Bella as a significant second-tier villain than the books had at this point. Similarly, the gravity on Julie Walters' face during the attack on the Burrow not only tells you everything you need to know about this warm, proud, fierce woman, but it also perfectly sets up THAT SCENE in the final chapters of the series.
It's not even the big-name Brit thespians doing this kind of sharp work -- though, seriously, additional kudos to Michael Gambon and Jim Broadbent, who tie for Best in Show here. But some of these teen actors are aces here. Evanna Lynch is once again the most perfect Luna Lovegood I could have ever hoped for. Frank Dillane -- son of Stephen Dillane (The Hours) -- freaked my shit out as the teenaged Tom Riddle, like Chuck Bass with dark magic. And I was completely in love with Jessie Cave, who played lovestruck Lavender Brown like a cross between Lisa P and Pepe LePew. In the best possible way.
Was It Just Me? (or Hey, Check Out All the Gay-Ass Subtext!): Seriously, though, I can't have been the only person chucking to himself at the sight of that delightfully blatant phallic imagery during all those Quiddich scenes. Particularly given that none of the early films have made the Quiddich broomsticks protrude so triumphantly from betwixt the wizards' legs. I mean, when Ron and Cormac are in tryouts, trying to impress Hermione with their respective prowesses? I'm not the sick one here.
Also, the whole "Harry laying out the bait for Slughorn, and by 'bait,' we mean Harry's fine ass" thing was pretty provocative too, right? It didn't hit me until Harry's conversation with Dumbledore when he actually said, "You said Slughorn would try to collect me. You want me to let him?" It was all History Boys from then on out. No YOU go write some fanfic!
The Kids Are All Grown Up: Before you get grossed out, this isn't an extension of the above point. This is where I have to tip my hat to the central trio of Dan Radcliffe, Rupert Grint, and Emma Watson, who have all really come into their own as actors. No, I won't be throwing awards statues at them, but given how remarkably green they all were through the first 2-3 movies, who'd have ever thought they'd be able to carry a movie this restrained on action. Clearly, Radcliffe has made the biggest leap (not surprising considering the master class all those weeks on Equus must have been), nowhere more evident than how genuinely funny he is (I'm thinking the scenes after he takes the good-luck potion, among others). Well played, Potter Kids!