I would have rather had Hagrid killed off than Sirius. Not because I don't like Hagrid (I do), but because I think she's done as much with this character as she can (though she may surprise me yet). In my opinion, Sirius was a much more complicated and interesting character, especially after finding out that he came from a family of dark wizards.
I don't know if the things you mentioned were actually "weak" plot points.
I was always conscious of the fact that Harry could go to Snape about the dreams especially since they'd been working on the occlumancy togehter. But knew that he probably wouldn't think of that since he and Snape are not exactly friends, you know? And I never forgot that Sirius had given Harry a way to communicate with him, but that Harry had put it aside and never thought of it again. I think Rowling did a good job of mentioning it once, having Harry swear to himself not to use it (because he didn't want Sirius to risk getting caught), and then forget about it himself. I think her intention was that readers wouldn't remember that in the midst of all the excitement.
There wasn't any need for Rowling to recount Harry's subsequent detention with Umbridge after the interview. It had already been established what happens in detention, so the fact that she gave him detention again was really all that needed to be said.
After looking into the pensieve, I think Harry was more concerned about the new revelations about his father than he was about Snape's feelings. I'm not saying that Harry won't feel some sympathy for Snape (and let it shape how he tries to interact with him), just at that moment it wasn't his primary concern. I don't see how Snape could ever offer real insight into Harry's parents and their past. Snape views Lily and James through a very narrow lens. He couldn't be objective and honest about James at all, because to him James was just a horrible bully. But Lupin, on the other hand, knew James' faults as well as his good side. Maybe even more objectively than Sirius did.
Yeah, the archway....the curtain.....no one ever explained it, so I'm holding out hope that since that's where Sirius went, he may be able to come back someday (somehow). I bet we'll learn more about this in book 6.
I don't know if there's much more to learn about Petunia. I was hoping something dramatic would be uncovered, but Dumbledore explained it that by taking Harry in that first night, she had agreed to protect him. And that the fact that she and Harry both shared Lily's blood kept that protective spell going as long as he was at Privet Drive. Maybe there will be more about it in the next book, but I'm not sure. I think Petunia's part in all of this was pretty much explained in this one. (Though it would be a nice surprise if there's more to her after all....)
Finally, about the holidays. Hermione and Ron don't always stay at Hogwarts during holidays. Sometimes one or both of them will choose to stay with Harry if they're all doing research on whatever mystery they're trying to solve (like in books 1 and 2). But I recall both Ron and Hermione going home every summer in the previous books, Hermione going home at Christmas in some of the other books, Ron going home for Christmas (in book one -- can't remember the others), etc.... In book four they all stayed because of the Triwizard Tournament, right? And Ron's family came to Hogwarts to visit.
And in book 5, Hermione spent the summer with the Weasleys because of everything that's going on! Voldemort's back, the Order of the Phoenix is up and running again and there's a lot to be done. It's unlikely she would want to sit at home while the world (and especially her friends) are in danger. Same thing with going skiing. There was more important work to be done. She wouldn't just walk away from that for vacation. As for her parents, who knows what they think about it all. But I think they've raised Hermione to be independent and make her own decisions, which she does.
As for Mrs. Weasley, she may not be with them every day but she's still a mom (with 7 kids to worry about!). Even if they're away at school I think she's till involved in their lives (and certainly concerned about them). She's had to deal with Fred and George getting into all kinds of trouble, Ron nearly being killed a couple of times, Percy turning his back on the family, Ginny being taken over and nearly killed by Tom Riddle, and more. Plus now we know she works for the Order of the Phoenix, so it's not like she's sitting home eating bon-bons all day, complaining about her kids.
Any other thoughts on any of this? I'm sooo thrilled to have people to discuss the book with.
p.s. heartmama -- I've read back over this post to make sure it didn't come across as rude or anything. I can't tell if it does, so I just want to say that, while I'm disagreeing with some of what you said, I mean no disrespect. And I realize that what I post is simply my take on the book -- not absolute truth! I don't want there to be any hard feelings on this thread if people don't see eye to eye on what they've read!