jrista, thanks for your insights into some of the science behind the sensor properties/issues!
Apologies if this is slightly off the original topic but I suppose it does relate to the advantages/disadvantages of APS-C and pixel density against the larger sensor formats.
Regarding comments made by jrista on the softness:
I started to feel a little bit the same way after I had my 7D for a short while. The thing I've learned, as I've continued to use it, is that the 7D, with a far higher pixel density than most other cameras (particularly full frame cameras), is very succeptible to softening by optical aberrations (in other words, its a hell of a lot more demanding on lenses). Where a 5D II/1D III might be fine with a lens wide open, the same lens used on the 7D at the same aperture would appear somewhat soft...and one thing the 7D does NOT do well is control noise in areas of smoother detail (particularly bokeh, but softness from optical aberrations heightens the effect of noise in detail areas as well.)
I recently learned that stopping down just about all of my lenses just a bit results in far sharper photos, and noise in the areas where it matters...the key subject(s) and the detail within them, appears far less noisy. I will grant that the 7D seems to drop the ball a bit on lower-luminance smooth detail...particularly out of focus backgrounds...bokeh tends to exhibit noise fairly badly when other cameras do not at higher ISO. Not really sure why...perhaps the smaller pixel pitch and surface area heighten the effects of photon shot noise...either way, its an area where improved Q.E. and better SNR in a 7D II could really improve things.
It is interesting to hear your thoughts regarding the affect of optical aberrations on the detail and noise in an image. I have a 7D and use it for landscapes quite a bit and have noticed, particularly with the EF-S 10-22 lens that it does struggle with resolving detail towards the corners. Now I realise that the high pixel density sensor of the 7D will likely show up any failings of the lens, and that with an UWA lens some corner softness is to be expected, and coupled with CA and possibly field curvature as well...... I usually shoot af F/8 but still struggle to get corner to corner sharpness... at high magnification anyway. Fine detail certainly seems to become 'smeared' towards the corners though not consistently, particularly at infinity focus. Maybe that is partly down to focusing and the distance of subjects from the camera, and maybe even my technique. I have also noticed some noise at low ISO in blue/dark sky areas.
None of which takes away from the fact that I think the 7 is a great camera, and certainly not worthy of the 'piece of crap sensor' comments dished out. If I could afford a 5D I would probably go for it but not everybody has that kind of money to blow on a hobby.
The issues you are running into are probably more the lens design than anything. For one, regardless of the quality, a 10mm lens is REALLY WIDE. Thats the same as the 16-35mm L II on a FF...16mm is REALLY WIDE. Rectilinear lenses bend light more and more as you approach the corners in an effort to "fit a spherical world into a rectangular planar world." The wider you get, the more extreme that bending has to be, and the closer you get to the corners of the lens, the more pronounced "stretching" and "smearing" your going to experience. Not much you can really do about that, its more a matter of physics than anything. On top of the physical matters, the EF-s 10-22mm, while it is a great lens, is not an L-series lens. Its designed as a high quality consumer grade lens, but not a professional lens. The EF 16-35mm f/2.8 L II, while its essentially the same FoV on a FF camera, is a better designed lens. It certainly has its fair share of distortion, however that distortion is better managed and controlled, and the softening effects as you approach the corners are lesser than with the 10-22. The 7D is probably enhancing this undesirable aspect of these two lenses, however there is more going on than just pixel density.
In your case, I would shoot at a smaller aperture. People are very afraid of "diffraction", however diffraction is only actually a problem once the airy pattern is more than twice the size of a single pixel (or, the airy disc, the central bright spot, is larger than a single pixel.) The 7D is diffraction limited
at f/6.9. You could stop down to f/14 before you really start to experience visible softening due to diffraction. I would say an f/11 aperture would be ideal for landscapes, assuming you were using a strong, stable tripod, and would help improve the softness in the corners. You could probably stop right down to f/16 if you really wanted to increase DOF, and you would only experience minor diffraction softening. The key thing is that the softening from diffraction is less than the softening from defocus. You might experience diffraction softening that was stronger than defocus issues at f/16, but its a tradeoff range. I have quite a few f/22 landscape shots taken with my Canon 450D, which has a larger pixel pitch allowing for about 1/2 stop tighter apertures. Even printed out quite large (I think the largest I've printed...on canvas...is 40x30), the effects of diffraction are more desirable than a soft ridgeline or OOF near-depth trees and flora. I think f/16 was the ideal landscape aperture on the 450D, and I would say f/11-f/14 is the ideal range on the 7D (if there is no wind, your using mirror lockup/liveview, and a cable release, you could probably get superbly short, deep DOF f/14 shots without issue.)