Brutal hours and the type of work is often random and sporadic. As a junior, it is very dependent on the quality of the manager since he can make or mar your time there. This is more than evident since a high-flyer will take you along and teach you a lot and a sedentary mule is going to make you insane with boredom.
Lack of choice on progression of juniors - you have to put your time in at each level and no real performance evaluation benefits; global locations can include undesirable ones