http://www.rei.com/online/store/Prod...cat=REI_SEARCH
L-bars are less likely to hook trees. In fact, they deflect you off of trees and protect your hands from strikes (if you're on the grips). Out in the open, they allow you to stretch out and get more "aero".
I'm not sure if anyone makes carbon fiber L-bars. My opinion seems to be a minority as so many folks are after "mini" bar ends that weigh less.