Nick, you're basically on the right track on all counts.
The only idea in your logic I'd refine is that it's really greater enlargement reveals flaws in technique. This is only related to pixel density in that people expect greater pixel density to allow greater enlargement.
If you make, for example, 24x36" prints from your D300 and a D800, and view these at the same distance, all things being equal I'd expect the D800 print to be more forgiving of flaws in equipment and technique despite the greater pixel density, because the larger FX sensor means you're enlarging less, and by extension enlarging your flaws less as well. The "all things equal" is a real rub here however, since the larger sensor will require larger lenses and be less forgiving of lens performance in the corners, and a million other details we could discuss, but the basic idea is that it's enlargement that magnifies your flaws, not pixel density.