-Call-ins. Everyone I have worked with, and myself included, hate the call-ins. Like many people have said, you really can't plan anything until you find our whether you're needed or not. It's annoying having to try to plan around them, especially if you find out you're not needed... the day is pretty much wasted.
-The hours. Hours are pretty inconsistent and can range from having a ton (to the point where you don't have any time for social things or studying-like during the holidays) to almost none at all (during slow Winter months, like mid-January to March).
-Floorsets. The set times to do these are just terrible. The hours may be good, yes, but it's long, hard work. Sometimes they are on certain days where it will clash with other things going on, like Superbowl Sunday and Christmas Eve. Many employees don't show up for the clashing nights, which makes them all the more difficult to complete them successfully and on time.