Let me start by breaking this down a bit. As an avid FPS gamer, I can say there are a few things a mouse must have to work in this sort of gaming environment. The most basics are the movement and the ability to fire and sight down the weapon. This leaves just about any mouse since somewhere in the 1970's to be on the list of answers. Over the years though, most users expect a certain level of "features" to already be thought of in the mouse before they would even consider buying it.
These days Teflon feet, high DPI sensors, comfort grip contours and material choices - they are all what we "expect" to already have been thought out. But what puts a mouse over the top for me are things like on-the-fly DPI changing, a lighting scheme that doesn't resemble a Christmas tree and functionality that is easy to use. On top of that a couple extra buttons here and there and one of the "sniper" buttons popping up on all the latest mice are all just icing on the cake with our latest example.
Del med dine venner