Think of it like this:
Diffraction is the maximum potential resolution at any given aperture, no matter what lens, format, or brand. It's a physical limitation, a ceiling, if you will, on how much you can get out of the lens. So if at F/8 diffraction says with red light you can only resolve X, then X is the maximum you'll ever resolve, even if the lens was perfect. (I don't have the time to do the math to tell you what X is at the moment, but you get the idea). Diffraction limits vary with the color (wavelength) of the light.
Lens aberrations tend to vary dependent on the lens. Most lenses tend to correct most aberrations better when they are stopped down a bit. Stop down too far though, and of course you'll be "into" diffraction.
Another way to think of it is that if you just won the lottery and went out and bought the latest exotic Ferrari (or whatever). The car has a maximum top speed - can't do anything about that - it's as fast as that car will go. That's diffraction. Your skill as a driver and/or the road conditions and/or the kinds of tires you're using and/or the weather are factors that will determine how fast YOU will actually be able to drive the car at any given time. It likely won't be at the top speed.
Having said this, don't get paranoid over diffraction. It's there, but it's not a binary thing where if you venture into the land of diffraction your images immediately suck - it's more a gradual thing, apparent in some subject matter more than others, and can be partially mitigated with careful de-convolution sharpening (focusmagic, etc).
Speaking personally, as a D800E owner who owns top tier glass and is rather picky, I realize there is a tradeoff between apertures that are "not in" the diffraction zone and having sufficient DOF for a scene as well as being at an aperture where the aberrations are well corrected and the corners are sharp. I tend to "know" my lenses, and as a general rule tend to shoot the wide angles between F/7.1 and F/9 if I can, and try to stay away from F/11 or beyond unless I need to go there. If I'm in doubt, I'll aperture bracket a scene so I can cover myself with choices.
-m