I think the Trio will come much closer together. They already have a very strong bond of friendship, and they are currently the only three who know about Voldemorts Horcruxes, except for Voldemort himself and possibly some Death Eaters. I think this will become important, as Harry is reluctant to tell anybody about them or about the prophecy. He didn't tell Ron and Hermione for a while about the prophecy, and only on Dumbledore's command about the Horcruxes. He also wouldn't tell McGonagall, who Dumbledore probably trusted a great deal. However, I think he will have to tell someone, sooner or later, and I can see him relying on Ron and Hermione's help a lot more.
As for the adult characters, I don't think Molly will change a great deal from OOTP and HBP. I think she's worried a lot by the events, but trying to carry on with her life, which means caring for everybody else and aiding them in their job. She's almost like a mum to the whole Order, and but she might surprise us all in DH. She could, if anybody threatens those that she loves.
I think the whole Weasley family will change, in fact. Percy may or may not apologise, but I can't see most of the family forgiving him any time soon. Fred and George will be forced to mature probably, they seem to have matured slightly in HBP, and I think this will carry on, but not a great deal, that would loose the idea of fun that they have.
Romance will definiatly form a bit part of DH, so many relationships were hinted at and developed in HBP that it would be hard not to include this. Ron/Hermione, Remus/Tonks and Harry/Ginny all spring to mind, as well as Bill and Fleur. Fleur isn't going to let anything stand in the way of her and Bill! I can't see Ginny being happy at being left behind, I think she will come along for at least some of the time, she'd been through almost as much as the others and is clearly able to cope. Remus finally gave in to Tonks at the end of HBP, but, much as I like them both, I can't see it being an easy relationship, or one with a happy ending. I hope not, but that's the way it seems to be leading. Hopefully, I'll be wrong on that bit!
The dark side. Snape has many more twists in his plotline yet, before he finally reveals who he's been truly working for at the end. I can't decide, but there'll be a lot more mystery before we find out anything. I can't see him surviving, I don't think there's anything for him to live for after the war. I hope Draco survives, I think he's been led to believe a lot of things by his father, and has been bullied into doing things he didn't want to by both Lucius and Voldemort.