What does this mean?
Snape is an important character in the books, sure, but he's not THE important character. Obviously, that's Harry. The most important other characters are Hermione and Ron. Just look at the way they're helping him out, being right there to back him up. They're going on the Horcrux Hunt (tm) with him, no doubt about it. And he'll have help from "unusual" sources: Dobby the House Elf, the spirits of his family (or at least his Prongs-Patronus).
Wandless magic is important, and so are the Horcruxes themselves (notice Harry is wearing the locket in both versions).
Overall, the tone of the books will be more hopeful than hopeless, with more focus on fighting-for-the-good-of-the-world than grim-struggle-against-Evil.
Snape's absence is key to me, though. We might find out what his big secret was, or we might not. In any case, he'll be around by the end of the book, but I bet he will have a relatively minor role (compared to his role in OotP or HBP), and I don't think I mind that at all. The less he's in DH, the less JKR can screw with his characterization and backstory, and the more likely it is that he will survive.
p.s. Notice there are no adults at all on the cover? Harry isn't going to win this war by organizing the Order of the Phoenix and using the Ministry to his best advantage. He's going on a much more private Quest, with his best buds at his side, and while Tonks and Kingsley and Lupin and the Weasleys will all be in there somewhere, probably helping out in background ways that JKR won't find interesting enough to tell us about, they won't be main characters after the wedding and after Harry leaves the Burrow.
p.p.s. And now we know Nagini is a Horcrux. ;-)