I am also curious to hear the answers to Marty’s excellent questions. I think this is where a sample image with EXIF data intact is worth more than a 1000 words! Being that you state you are disappointed with the loss of distant detail, it sounds more of a DOF issue than an unsharp lens issue.
But I would also add that a lens can only truly focus on a single plane, and everything else falls under relative sharpness. Relative sharpness and DOF is highly dependent upon the amount of enlargement and viewing distance. If you focused upon a foreground element, given sufficient aperture the background will fall under DOF or relative sharpness. But DOF tables generally assume an 8x10 print (bit smaller than A4) viewed at about 18 inches. If you print larger and view it at about the same distance, what appeared to be relatively sharp before can look less sharp.
While I had a D80 and 18-135, I didn’t really use that lens much because I acquired the Nikkor 17-55 f/2.8 shortly after getting the D80 and mainly used that with the Nikkor 70-300 VR as my main lenses. So while I could provide landscape images taken with the 17-55, I don’t think it would be a fair comparison to images taken with the 18-135.