I just started riding fixed this spring. I rode a unicycle a little bit in high school (5+ years ago) and I can tell you the following challenges that will still come up, minor as they may be:
1. Pedal strike. Is nearly impossible to do on a uni, but I did it on the way to work this morning. Just be careful.
2. Toe overlap. Again, with no front wheel, it can't happen. Just be careful when turning at slow speeds.
That's it. You'll probably be doing no-handed trackstands, backwards figure-8's, and wheelies for blocks now.