Most common keyboards use cheap rubber dome switches as opposed to Mechanical Keyboards which use high quality mechanical switches. Mechanical switches can vary quite a bit, but most contain individual springs and are consistently higher in quality than rubber dome switches. The keyboard framing, key rollover*, printing / etching, key plastic, PCB board, LED lighting (sharpness and brightness), and a slew of other features are also generally far superior in Mechanical Keyboards.

The key thing to remember is that Mechanical Keyboards just feel better and will make you a more efficient gamer / typist.

* How many keys you can push at once and still have the computer recognize the key press.