Just finished it (seriously--like five minutes ago). I can't remember what order your comments were made in, so I'll address them (and my own) in the order I think of them.
1) Umbridge didn't actually get off a cruciatus curse, but she did admit to attacking Harry with Dementors and that, at the very least should have gotten her thrown into Azkaban. It was very odd that Rowling seemed to forget this, as she's usually very tightly structured. On the subject of Umbridge and Fudge, however, I'd like to point out that I'm very glad they turned out to not be Death Eaters--it was refreshing to see another side of the war instead of just the good/evil dichotomy. The MoM was a very satisfying villain with a different set of priorities and methods than the other villains we've seen.
2) Sirius dying was kind of cheap, but kind of important, I guess. He's certainly not a major character (for the first 400 pages I was certain Dumbledore would be the one), and I think a lot of the sympathy we're supposed to feel for him was forced on un via Harry--I never felt connected to him, so I was supposed to feel bad because Harry was connected to him. Maybe that's just me, though. In the end, I think it's important for the direction of Harry's character that he doesn't have the father figure to help stabilize him.
3) The characters were so accurately adolescent that it was sometimes painful to read. I wanted to shout at the book sometimes because the characters were so sullen, or so hot-headed, or so bipolar, but I had to remind myself that that's exactly what 15-year-olds are like. The only character I thought was undeveloped and false was Cho--I was actually convinced that she was a metamorphmagus in disguise, trying to spy on Harry, but after their second break-up I dropped that idea. In the end she turned out to just be an over-emotional teenage girl, which helped me accept the character somewhat. As Lupin put it, "everyone's a jerk when they're 15," so the fact that Harry didn't learn anything or talk to anyone or resolve his conflict with Snape was fine with me.
4) Speaking of Snape, I can't tell you how glad I am that he continues to be a fascinating and complex character, both noble and petty. I have an unexplainable attachment to him, and I root for him in every book. I do wish we'd gotten to see a little more of his undercover work, though. And yes, I'm sure that wasn't his worst memory.
5) I agree with EUOL--the "startling revelation" about Harry's scar was a huge letdown. Tell us something we don't know, Rowling. The meaning behind their relationship is so obvious I actually thought they'd already explained it.
6) While I continue to say that the second book is the worst (I actively disliked it, whereas I've enjoyed all the others), I have to agree that this one was a little weak and showed some poor writing. The book was an endless cycle of "something happens, we reflect on it," over and over, as dozens of tiny one-shot conflicts pop up and get smacked down a few pages later. This was also the first book without a twist ending (one of the reasons I kept suspecting Cho--I tell you, the concept of the metamorphmagus was a gun on the mantle that bothered for all 870 pages), though in some ways I was glad, because I'm really get sick of twist endings in modern media.
7) Stuff like Grawp (and, I suppose, the metamorphmagi) was obviously put there as hints and set-ups for future books. I have to say that I'm really intrigued by the steadily growing themes of racial equality, and I'm dying to see how she wraps it up.
The romantic tension between Ron and Hermione was only addressed once, when she kissed himon the cheek, and I was really bummed to see Rowling ignore it. Of course, both of their characters kind of got neglected this time around, so I guess that explains it.
