What? An "M" rated game is "made just for young little kids", you say?
Consider the controls used in "Saints Row" for shooting while driving. You drive the car with the left stick and aim the crosshair with the right stick. Because of how the controls are mapped, you can do these activities at the same time, i.e. steer, aim, and shoot. In "Grand Theft Auto", I seem to recall having to hold down a button to do some sort of drive-by, and by doing so, I lost the ability to control the car, or alternately I couldn't really aim other than by angling the car such that the car windows were perpendicular to the target.
Another example is the map system... In "Saints Row", the maps are colorful and have nice icons denoting where key things are. It's easy to set route markers on the map and then use these in the main game to get to locations. In "Grand Theft Auto", the map is ugly and is a pain in the ass.
I guess ultimately it comes down to preference, sort of like whether one prefers the multiplayer play style of "Halo" or "Call of Duty".