I have been researching crop vs FF since I am lusting after a full frame camera. I was looking at a comparison with a bunch of images where you couldn't tell the difference as the crop was shot to mimic the FF.
One of the comments a person made was making a crop mimic a full frame is bunk, the images should be with the same lens and setting (aperature speed etc) to show the difference.
Well I wanted to say something to the poster. Of course a crop is going to be different than a FF that is why they have 18-135 lenses rather than 28-200's. a crop make an 18 virtually the same as a FF 28mm.
I would be like me trying to decide on the best car to drive 1/2 mile to the grocery store through a downtown city, a Ferrari or a family sedan and then complaining that you made the Ferrari drive 25 mph through the crowded streets when it can go much faster...
Maybe not a good analogy but if the crop sensor could only use the FF lenses then you may have an issue especially at wider shots but that is why they have lenses for the crop sensors. To me any shot you can get with a FF you can also do with a crop sensor. The difference as I see it is dynamic range and low light sensitivity.