I guess I'm just a total Pollyanna because I'm seeing the glass half full. I'm assuming that the human and political aspects of GOT will all still be there along with the magical threats and mysteries. If they aren't, it won't be worthy of the label "Game of Thrones". We'll also have new takes on the ancient families we've been following since GRRM created them in the 90's. We all have assumptions about Bran the Builder, Lan the Clever and Duran Godsgrief, because of the Houses they founded. If this show is true to it's parent show, all those assumptions will be turned on their head. Maybe Lan the Clever will be one of the protagonists and House Casterly will be the big bad. Maybe Bran the Builder is drunkard everyone thinks is a lunatic going on about White Walkers.
The big elephant in the room for any prequel series is that everyone knows what the outcome is. We will know that somehow the end game will involve a final pact with the Children and building the Wall. We'll know that the Stark, Lannister and Duran dynasties will be founded.
I'm also personally grateful that it's not a Targ-fest, as GRRM has that covered with all his detailed histories and side stories. I'm also very relieved it isn't Robert's Rebellion, because that is covered adequately in the novels and the series. I feel that ground is well tended, but the history of Westeros before the Targ's is much more wide open. (That being said, I'm also totally open to a future series or movie or novel about the Doom of Valyria.)
Editing to add - Casting will be everything. I wonder if once they start casting if we'll know who the lead characters will actually be.