I've just started GM'ing WFRPG over the last few weeks. I'm running the "Elder Eye' adventure from Tome of Adventure, and so far two things jumped out at my group as sources of concern:
1) An average difficulty challenge means two challenge dice, which can be pretty brutal all by themselves. 5/8 of the die has failures, and 1/4 has two failures. It seems quite easy to get three failures. On the other hand, characterisitic dice are not nearly as generous. An average characteristic is 3, and even with the extra die, the probability of all successes getting washed out seems to be over 50%. 2d difficulty is actually quite rough. Stance dice can make a difference, but stance dice (apparently) don't come into play during story mode.
2) Pancake factor. There doesn't seem to be much variation in damage output. Usually, three successes are needed for either a player or creature action to inflict more than basic damage, and it felt like a struggle to even get one success (or boon, for crit purposes). When damage is simply a matter of weapon + strength, and soak is just a matter of armor + toughness, it seems pretty easy to wind up with attackers that wash out damage. Over time, players will probably upgrade their armor to even better gear, and monster damage will stay the same.