I know what you mean about a 'sense of dread'. Knowing that someone fairly major is going to die in book 5 (though, I'm naturally curious as to who it is) isn't something I'm looking forward to.
The tone has changed dramatically for Harry and his friends, and that'll certainly be reflected in the 5th book. Harry will have had all summer to reflect on things, and I think it'll all be on his mind when he goes back. Being 15 (as that isn't an easy age anyway!) will be a challenge right there.
I know there's been talk of romance as well book 5 and beyond and provided J.K. handles it as I believe she will, I won't mind it. It wouldn't surprise me that people would be dating by book 5, but I certainly hope the books don't become all about that, and I don't think that they will. I am concerned that Ron and Hermione may date, break up, and then Hermione might date Harry, causing bad fall-out between Harry and Ron. That's all speculation, but I didn't like it when Harry and Ron weren't friends, anymore than I enjoyed the Hufflepuffs (in book 2) talking about Harry behind his back. Granted, I remember that kind of thing all too well at that age, which, while making it realistic, also brings back some not-so-fond memories.
Part of "dreading" book 5 is as simple as knowing I'll read it pretty quick and there won't be a "book 6" for a LONG time!
That's sort of silly, but I have a feeling I'll read book 5 several times. Mostly, I'm looking forward to it, but as it will be darker than book 4, while that'll be interesting, I think I'll have my box of tissues by me when I read
. . . shoot, I needed them when Cedric was killed in book 4 and all the aftermath after that!