I think comparing tW3 combat to a more shooter style is rough, because the skill sets required are so different. and fast paced doesn't necessarily mean harder. we don't know how lethal YOU are, or enemy group sizes, etc. the extra mobility is probably how they're balancing the combat.If I remember right, it was said somewhere based on the E3 demo that the difficulty is higher than in TW3, as the combat is fast paced and the player's character dies in 2-3 hits. And avoiding hits might be harder, too, since you do not have Quen and the attacks are mostly ranged. However, this is also an early build, the final version will likely be balanced so that the default difficulty feels about right for the average player.
and yes, I'm sure the default difficulty will be playable for most people.