There's a pretty big difference (legally and ethically) between making a game that has similar mechanics to another game and just outright ripping out graphic assets to place in your own game.
It doesn't really matter if the graphics are pixel-by-pixel the same, modifying them a bit to make them slightly different doesn't protect you. See: EA's lawsuit against Zynga over The Ville which Zynga had to settle.
The problem with game mechanics as with other ideas is taking them and present them as yours or as new, even if you didn't add anything much to them. This is what I think people refer to as "ripping off" a game mechanic. The other case of "stealing" a game mechanic is perfectly acceptable in the collective mind. Nobody complains about Half Life, Unreal or a first person shooter game that ripped of the main mechanics of a FPS.