I used to be rank 90 in the world on Black Ops on 360 USING A GAMEPAD, so please understand I am not biased when I say that, while a gamepad can be extremely good in the right hands, in general a KB+M have a higher skill ceiling and finer control, especially in terms of aiming. The analogue nature of the thumbsticks on gamepads do allow for easier modulation of your movement speed which is useful in some games, however there are also a lot of situations where the fact you have 5 fingers with which to press 5 keys at any one time is a much larger advantage. For example, in CoD on Xbox360 you can't crouch/prone at the same time as aiming, or if you use tactical layout then you can, but you forfeight the ability to melee while aiming. With M+KB you can do both easily. For these reasons FPS games are generally much better played with M+KB.
Same goes for RTS games for obvious reasons, I'm sure no-one would argue otherwise.
Some genres which are genuinely better on a gamepad would be most platformers, racing games, flying games (not full flight sims - those are better with M+KB).