As an added incentive to those on the fence or waiting for a sale, Steam is keeping track of the number of preorders. Once certain intervals are reached, rewards will be unlocked for the early adopters. The first level will provide you with tools to customize your XCOM soldier, the second level opens up some goodies in Team Fortress 2, and the third level gives all prepurchasers a free copy of Civilization V.
As of right now, the counter is at 28% of Reward 1, so if you’re going to pick up XCOM: Enemy Unknown you might as well get on it and get everyone closer to some cool free stuff.
Full details about the game can be found here.