I'm just going to speculate on this by pretending no one else has said anything (it'll be easier this way; don't ask, I'm just strange), so some things are going to be obvious because others have already said them ...
1. I'm thinking that maybe those who become ghosts are the ones who aren't ... ready for death. Sirius, for example, knew what he was chancing when he joined the order, and
was ready. Moaning Myrtle (who had simply opened her stall door intending to tell Tom Riddle off for being in the girl's bathroom) and Nearly Headless Nick (who had been beheaded, though for reasons unknown to us, I'm guessing he had done some sort of crime) are examples of those who
weren't ready.
2. One of the reasons I always believed Dumbledore never made Snape the DADA professor is because he'd be hard put to find a better Potions master. See, even though the DADA post has not had very good teachers (with the exception of Remus Lupin and Barty Crouch (I know he's mad, but he did teach them a lot)), lots of people know Defence because of Voldemort's reign, but I doubt many know Potions as well as Snape could because it can be very, very, very difficult, and Defence is just - easier - I guess. For example: Harry can do a Patronus Charm, yet his potions marks still aren't their best, and even in his OWLs without Snape, he felt that "he might not have achieved a good grade". Another thing is Neville, who is hopeless at Potions but very good at Defence, you know, with the DA and everything ...
3. I see this as a repeat of PoA, when everyone was worried about Sirius trying to kill Harry. Most likely, Dementors won't be guarding Hogwarts because Dumbledore hates them, especially since they tried to Kiss Harry, and besides, they're on Voldemort's side. But I do believe various members of the Order (how about Kingsley Shacklebolt? Now that Sirius is gone ...) will keep watch over him anywhere he goes, but in secret. Why in secret? Because Harry will yell at them, of course. Something along the lines of, "I CAN TAKE CARE OF MYSELF!". ^_^ j/k But I'm telling you, Harry is
scary sometimes ...
4. The Dursleys will be nicer to Harry. There's no other way. Since the threat by the Order members, Petunia would freak that they might come marching up the garden path, as Harry speculated in OotP, and Vernon has to listen to Petunia. Dudley would just steer clear of Harry.
5. Harry is definitely darker, and will grow to be darker in the next books. However, I don't believe that Harry would become as evil as Voldemort. He might just possibly learn a few Dark Arts spells, but probably won't use them on anyone but Voldemort. Does anyone but me think that Expelliarmus will become very useful, since Harry's and Voldemort's wands can't fight against each other?
6. I was wondering that, and I was also wondering why Sirius didn't ask Harry to use the mirror when they were fire-talking. Maybe Harry thought that if he just touched it or something, it would tell Sirius he was in trouble, because he never actually opened it before Sirius had died. Or maybe he just forgot about it. Lots of things happened between the time at Christmas when he first got the mirror and when he wanted to talk to Sirius.
7. Nope, nope, and definitely not. I believe it was near the end of Ootp: "no matter what Dumbledore said, he would never forgive Snape, ever." There is the slightest possibility that he might go against his words, but I'm supposing that his hate for Snape goes down far too deep for forgiveness, at least until Harry gets over Sirius, which I personally think will take a really long time.
8. There's the Trio and Draco (Gryffindor and Slytherin), but that's about it. None of Ravenclaw, Gryffindor, or Hufflepuff hate each other, and given the chance, they could become friends. Indeed, some of them already are. Ginny and Michael Corner went out, then Cho likes Harry, despite their fights. Then there's the DA, which already did bring all of the houses except Slytherin together. I suppose that Slytherin's a hopeless case, but maybe in the next two books some of them will be nice or something ...
(don't know how to answer question 9, and it's getting really late where I am ...
)
10. I think that Harry does think his father was an idiot, as he said, but after his chat with Remus and Sirius, he realises that they're telling the truth. He also grinned when Ron reminded him of James, which means that he's finding it funny, how arrogant his father was. Or maybe it's just me. As for James and Lily, I do hope we hear more about them, and maybe the rest of the Marauders, as well. It'd be interesting to see if Harry turns to Remus, because he's the closest connection to James now that Sirius isn't there. Of course, I've always been a fan of the MWPP (Moony, Wormtail, Padfoot, and Prongs) world ...