I've just finished re-reading Harry Potter and the Order of the Phoenix (OOTP) and Harry Potter and the Half-Blood Prince (HBP) in preparation for the upcoming Harry Potter and the Deathly Hallows (TDH):
I'm most interested to find out what becomes of Snape. I think it's interesting that only two of the books have been titled around a single character: Prisoner of Azkaban and Half-Blood Prince. In the first, a seemingly evil character turns out to be good, and in the second, a seemingly good character turns out to be evil. Or does he?
I think you could make the argument that Dumbledore and Voldemort are mirror images of what Harry could become. One is absolutely good and one is absolutely evil. And along those same lines, I think that you could make a similar argument for the contrast between Sirius and Snape. One is a pureblood and one is not. One is good mistaken for evil and the other is evil mistaken for good. One wants to help Harry succeed and one wants to see him fail. And they both see Harry as an extension of his father, James, who one loved and the other hated.
I think that that argument is little too simplistic, though. Snape has had multiple opportunities to kill Harry and he has not taken advantage of any of them. As a matter of fact, in several different situations, Snape has actually saved Harry's life. He told Bellatrix at the beginning of HBP that he saved Harry from Quirrell because he didn't know that Quirrell was host to Voldemort, but why should that have mattered? If Snape was really on Voldie's side, he could have just done nothing. It certainly wouldn't gain him any points to save him – except with Dumbledore, and it wouldn't cost him anything to let him die.
At the end of OOTP, Snape chose to inform the Order that Harry and his friends were headed to the Ministry of Magic thus helping to guard Harry's safety and the safety of the prophecy. Again inaction would not have been questioned. He could easily have claimed that Harry didn't try to warn him or that he misunderstood the warning. Instead he passed the warning along and probably saved Harry's life.
Finally, at the end of HBP, Snape once again saved Harry's life from the other Death Eaters. And not only did he prevent them from cursing Harry to death, he prevented them from cursing Harry at all. He refused to fight Harry, and he refused to allow Harry to use any unforgiveable curses. In fact, when Snape told Harry that he would never win this fight because he wasn't a good Occlumens and he couldn't perform non-verbal spells, you could read that to be a tip rather than a jeer.
A lot of people, including Harry, believe that Snape was actually a double-agent and that he is in fact evil. I tend to not believe that. Dumbledore was not a fool, and exposing Snape as truly in league with Voldie would cast doubt on all of Dumbledore's past decisions. Harry believes that the reason Dumbledore trusted Snape was because Snape was repentant about the death of James and Lily Potter, but I think that there was a much better reason for Dumbledore to trust him. First, Dumbledore never actually told Harry the reason that he trusted Snape. He just told Harry that Snape's guilt over the deaths of Harry's parents was a turning point. Second, Dumbledore didn't trust Tom Riddle after any of the times that he expressed remorse, so why would he have trusted any of the Death Eaters on just a statement of remorse. There was bound to be a better reason.
I think that there is more of a connection between Harry and Snape than we have seen so far. I think that Snape has been intentionally mean to Harry to make sure that there is no possible way that anyone would think that he wanted to help him, but I think that Snape is going to be the biggest help to Harry when it comes to defeating Voldie. It's already been foreshadowed in HBP as Snape was the one that actually helped Harry continue in Potions by taking a new position, helped Harry succeed in potions as the unknown Half-Blood Prince, and helped Harry survive to fight again as he was leaving the castle. Harry is going to need Snape's help to accomplish his task and Snape is going to provide it.
I also have a couple of other kind of random thoughts. First, I think that there is more to the connection between Fawkes, Dumbledore, Harry, and Voldie than any of us realize. We all already know that Fawkes gave two feathers for wands and that those wands belong to Voldie and Harry. We also know that those two wands can't be used against each other as a result which makes it interesting to try and figure how Harry is going to kill Voldie (I also assume that Harry won't use an unforgiveable curse as that would be bad form for a good guy, don't you think?). More than that, though, I found it interesting in re-reading OOTP that when Harry recounted to Dumbledore that he had told Scrimgouer that he was Dumbledore's man, Fawkes sang and Dumbledore cried simultaneously. That got me to thinking back on how Fawkes had been the one to assist Harry when Dumbledore couldn't in Chamber of Secrets and how Fawkes had been the one to help Dumbledore communicate with Harry, Ron and Hermione in OOTP when Dumbledore couldn't be there. Finally, the most obvious connection of all is the fact that Dumbledore's patronus is a phoenix. Of course, how this connection is going to play out is anyone's guess, but I think it is interesting to note that the phoenix is legendary precisely becuase of its ability to survive death. In fact, death is a regular part of its life. Voldie has already resurrected from the dead once. Harry has already survived death once. So what does that mean for Dumbledore in the upcoming novel?
Another random thought is that it must be important that Harry has his mother's eyes. I can't tell you how many times I noticed that that was mentioned in just these last two books. I'll bet it's even more in the previous ones that I didn't re-read. How does that play into the situation? And why does it keep getting so much mention?
I just had to get all of this out before the book is published. Let me know your thoughts if you are interested.