Stealing people's games.
I'm so freaking sick of seeing people on Concepts take other people's game, say that it's their own, and put it on Greenlight. First, some 10-year old kid wirh Slender, then some other person with GTA5, then someone stole Brutal Doom and put it on Steam. I thought Concepts was for posting concepts of YOUR game, not games you want on Steam, or taking someone else's game. If people can abuse it to such an extent, than I don't even want Concepts here. Gosh darn, man. Heck, the reason why Valve made you have to pay $100 to get your game on Greenlight is so that some 12-year old with an Xbox-Live account can't steal other peoples' games.