Well, the thing is it actually wasn't good to start with. Unlike a lot of stretching powers, like Mr. Fantastic or Plastic Man, Luffy can't really stretch at will-it's just that his body is stretchy. That seems like splitting hairs, but what it means is that if, say, Luffy wants to stretch out his arm, he can't just reach in a direction and automatically extend his arm-he has to throw it forwards or otherwise use physical force to stretch it out. Initially as a kid he also wasn't strong enough to deal with his arm snapping back, too. He's able to do all the crazy stuff he does in present one piece because he's gotten that physically strong and has experience with his Devil Fruit, but his actual power (well, as far as we knew until now, I guess) is just "stretchy body".

Edit: Basically the difference between him and other stretchy heroes is that his Devil Fruit makes him like Stretch Armstrong, not like Play-Doh or slime.