As far as airports go, there's not much for me to complain about in Charlotte.
There was a period of time that TSA lines were incredibly long, but more recently they have fixed that. It doesn't take me more than 20 minutes to get through now, even on a holiday.
I think their food selection could use a little improvement. The other big annoyance I have is the lack of outlets at the gates. From what I've read they are fixing this with the new terminal, but the existing ones are bad in this regard.
If you have a long layover, you should definitely check out the rocking chairs as well as my favorite froyo, Pinkberry, in terminal D.