Honestly, I'm not very enthusiastic about this prequel. The parent show couldn't even manage to wrap up it's final two seasons with ten episodes each; be them short on story content or whether they just want to wrap it up quick is beyond me, but in my mind, this doesn't bold well for a prequel/sequel show. It's obvious why they're doing it; Thrones has made a lot of money, and they know they've got a financial success here (so why not extort it), but yeah, not overly enthusiastic.
My other problem, I think this'll end up just being another medieval show. Many fans of Thrones are in this (in part) due to the mystical nature of the show. If the prequel doesn't remotely touch on that (at least until the final chapters), I fear (maybe) they'll lose a few ten-thousand viewers.