Sticker shock of renting an apartment because on campus housing wasn't available, in a city that was supposed to be "affordable", after living in and paying for a house mortgage. The only way you can figure this out correctly is to actually call or visit housing in the area or use Craigslist.
Renting a room (rather than own apartment) isn't so bad, even after living by myself for five years.
Sleep Pillow is a wonderful app for getting rest when living with roommates.
How difficult it would be to transition from a full-time job to no job and full-time graduate (MA) school. Losing that sense of fulfillment and usefulness that comes from having a job/career.
As someone else mentioned how much sitting I do, sitting in class, LOTS of reading, never ending reading assignments, and some writing. The outside of class work read load is much larger than undergrad.
Lack of finals or any type of exam (at least in my field, international studies) for classes. When people ask how I'm doing in my class, uhh...I don't really know because the class only requires a final paper and presentation, which I won't get a grade for until the class is over!
Comparing my undergrad institution with my grad institution...trying to not make those comparisons.
How quickly my interests and career goals could change once I actually started.
Advisors are not always very helpful. My assigned advisor kept letting me know I was free to choose my own advisor, well thanks, but I don't really know anyone else here yet.
Disappointment in my grad school and program, i.e., program not as diverse or flexible as originally thought (despite checking the course schedule & descriptions), limited availability of classes (summer, fall, spring), decrepit facilities compared to the cost (it's a good idea to do a full tour of campus, back to comparing undergrad).
How many times I would think about quitting.
How much I would enjoy the class topics, discussions, and readings. How much I have learned in a short time and how much my original views have been turned on their head way more than as an undergrad. That if I wanted to do a thesis (not required in program), I now have plenty of ideas to choose from.