I was talking about the shape, though. I could stick the best mouse sensor in the world in a red clay brick and that mouse wouldn't be a good mouse. Basically, there is a whole lot more to a mouse than the sensor.
You want to look for good build quality, with good buttons that won't wear out, a shell that won't crack or wear, and a good surface.
You want it to be comfortable on your hands, and some surfaces will get sweaty and greasy. Some paints will also wear off of mice, you can usually see if this happens by reading some reviews.
DPI is not the sole measure of a good sensor, as it is all preference. Many FPS gamers play with low DPI settings in order to achieve higher accuracy, as it slows down the mouse to reduce shaking.
Something else to consider is the surface you play on, a good mouse pad or mouse mat can be very useful. A cheap one that's curling up, rough, and slides around will completely negate the value of a good mouse. Spend a little more on a good one like a Steelseries QCK(there are a lot of versions, I have the +). You can get a giant one so that you don't need to lift your mouse, or a smaller one if your desk is cramped. Some people prefer hard mouse pads that the mouse runs over quicker.