The bad things:
Sleep Inertia- this is when you feel groggy or kind of disoriented when you wake up from a nap. (this is my issue if I sleep any longer than like 10 minutes)
Nighttime Sleep Issues- if you do nap, keep them to a shortened time. Naps during the day can play a part in continued nighttime insomnia. Also if you take longer naps during the day it can inhibit any sort of good sleep that you could get at nighttime.
If you are going to nap, there are some things to consider. Make sure to keep them rather short... up to 20 minutes. Also, make sure to take a nap in the early part of the afternoon, like before 3pm. And as with almost all sleep, make sure you are in a dark, cool room with minimal distractions.