Remember, if you shoot with a crop sensor camera you have more DOF than a full frame camera if you were to attempt framing the same shot with both types of cameras... Take the same 50mm f/1.4 lens and put it on a Full Frame D700 and then put it on a Crop D300. Assume you are shooting at f/1.4. Because of the crop factor on the D300, you need to step further back from your subject which gives you more depth of field. The same composition on the D700 will yield a background that is blurrier than the D300. This is one of the main reasons people prefer to shoot with a full frame camera.
Photo credits: Serban Enache, Danijel Micka.