Oh
The comparison to Ysera is a good point, but Nefarian's more of a direct threat than her. You don't want to start taking 8 damage! I think Ysera has more of a control aspect (no pun intended) to her while Nefarian is more direct pressure. Also, since this is going to be from an adventure it'll be a far more accessible card for most players!
I'd just like to strike a blow for Dragon's Breath.... Sure Fireball,and most of the times,also Frostbolt are better options,though if you used Flamestrike on a full board this card is 4 Damage for 1 or even less 0 mana so it's not that bad... I mean 4 Damage for no cost isn't bad and neither is 4 damage to a minion that Flamestrike couldn't kill,since minions that could survive 8 damage can be counted on one hand......