Alex
Probably the weather. Walking to an 8 AM when it is less than 8 degrees is never fun. The thing people forget to mention when complaining about the weather is the fact that all freshman get a years worth of free bus services, which usually run every 10-15 minutes from multiple places around North Campus. If you learn the bus schedule you can usually avoid the worst weather Ithaca has to offer.
K
The workload gets a little intense, there's no denying it. When it's not final or prelim season things are definitely manageable, but when it gets to studying crunch time everyone's nerves are a bit on edge. Cornell students are competitive and they want to do well, so they'll usually put in the time to study, sacrificing a few days to the library for the sake of a decent GPA.
Plus, the weather gets a bit iffy. This was one of the things I was most concerned about before coming here, but for the most part it's severely over-exaggerated. I've developed a healthy respect and appreciation for sunshine; coming from Southern California I've been used to perfect weather my entire life, but I've found it quite easy to adjust. So don't worry. There are a lot of foggy days and rainy days, but the bus system on campus is excellent and it's not that bad.
Zac
Having a lot of work to do to be honest. If someone is complaining about something, it will be an upcoming test or paper. Also, as of late, restrictions on the Greek system have been getting people angry.
Bryce
Difficulty. The work is really hard and the tests are harder. It can be stressful, but its definitely doable. It's not uncommon for a test to have a mean in the 50s or 60s, but its not a problem since you can do very well in a class and not have to get what initially appears as a bad grade.
I find that people often question why the scores are so low, asking if the professor taught how to do everything. Yes, they teach you everything! The questions on tests often require some really intuitive problem solving that ultimately separates people into those that memorized how to do questions and those that understand how and *why* everything works. It's a good system that encourages you to really apply what you've learned to more than basic textbook questions.
Sam
The work (especially during test periods) and the weather (especially during January/February).