hmm. Let me see, here are a few.
While working for a small design studio with only a few employees: clients' budgets were small and I only had limited time to do the work, could not finish the project to my satisfaction/basically was forced to do work that was not up to my own standards. Moved to a big firm was a welcome change, here there was lots of creative freedom and time to come up with great work. Here however the process is much longer and there are a ton of people involved in the corporate hierarchy. Some are great and some are tough to deal with. From time to time there is a decision maker person who does not care about the creative part of the project they have other agenda (market research or focus group results) they have to deliver on and the work had to yield to that. I have learnt to with with everyone, be flexible, have a great relationship with my art director who could stand up for the creative work when it was necessary. Overall I had to realize that it is not about me and not to take criticism or change orders personally, not to be attached to my own creative ideas so much (there were plenty of times when my ideas were very much appreciated).
Now that I am out on my own the challenges are being responsible for getting new clients in, having to deal with the business part of things (taxes, accounting, billing, setting my own prices, paying studio rent), dealing with all parts of client relationships, manage the legal side of the creative business (licensing, copyright issues, writing up work contracts). Pretty much learnt as I went along, use software for accounting, hire a professional for taxes, learnt about legal stuff and professional practices from books. There are professional organizations with local chapters that have memberships for students as well with online resources too. AIGA and GAG.
Good luck, enjoy your creativity!