Shields.
They may be a nice extension of tactics to rush that camper, and they might be a good way to get through the vent in cs_assault, but they are just way too bugged, and have been banned from all professional cs leagues for a reason.
It does not matter whether the enemy has the shield flipped in or out, if he faces you or not, it is just pure luck to kill that guy behind the shield. It might be fun to rush the ever-camping Seth with it over and over again (gg anarki), but it's just an unbalanced piece of crap.
I just came back from a game of CS, and there was a guy hiding behind the shield, but he was not looking at me, but at that guy on the other side. So I stand on the right of the shield CT, who is looking so that the shield covers nothing of his body from my point of view. I pull out the AK, aim nicely at the head and give it a burst of fire. As a reward, I see some sparks and get the nice sounds that are played when bullets are deflected by metal.
Therefore, I think shields should be banned from the CS server was well, as in disabled by admin mod.