What has to be emphasised is that Alamy doesn't care whats in your pictures. From dog poo to celebrities - anything goes. If you think somebody, somewhere might some day need that image - go for it. But on the other hand, Alamy insists on digital excellence, so that a picture is not rejected by the customer because of perceived faults. That means that there are a few areas that are strictly no-no. These are:
Soft or lacking definition (SoLD). Because of the anti-alias filters on our sensors this is always controversial. Alamy knows the difference between that phenomenon and ever-so-slightly out-of-focus or slight camera shake. As an example of what can happen here's a story. Six months ago I bought new an Olympus EPL-1 micro four thirds camera, complete with kit lens at a really good price. But I noticed that I was having to reject more than a few images due to SoLD. I reckoned that this couldn't be down to camera shake by me when I checked the IPTC data - my speeds were high enough, my f stop was f7.1, as always. I dug and dug on the Interweb and finally came across a site which had also found this and went to the trouble of testing three individual lenses. They discovered that the construction of the lens caused a wobble if used between 1/100 and 1/200 sec and thereby imparted camera shake to the image. The shake was really miniscule but to the trained eye (as in Alamy) it was noticeable. I bought a third generation lens at the same price as the original entire camera and the problem was solved. The manufacturer has never owned up to this fault, DPreview never caught it but it was noticeable when the image was forensically examined at 100%. You can be sent to the submitting sin bin for a month by Alamy for a number of SoLD submissions. Ain't worth it.
No sharpening should be applied to an image, except perhaps for the default Photoshop RAW processing. And sharpening artifacts can occur by other means in the processing, such as over saturation or colour removal at the Tif stage.
I only use the lens' "sweet spot" when shooting. Each lens on a 35mm style digital camera has a particular f stop at which the lens is sharpest, usually f8 or thereabouts, due to refraction within the lens. This is, in my opinion, extremely important. So much so that my aperture setting is permanently set and the ISO setting at Auto for non-studio shots. My cameras are very good at noise - up to 1600 ISO is no bother when the exposure is good. (Nikon full frame 12MP D700 being the other camera). If I have a depth of field problem in the studio, I take a number of shots at different focus points and depth of field blend them in PS to achieve sharpness throughout the image, such as in this one:
Chromatic Aberration: All lenses give this to a greater or lesser degree, some saying that more expensive and prime lenses being better. Has to be removed. There are various tools and methods, which I'm sure you know. I tend to use Replace Colour , if necessary selecting the areas, so as not to damage other areas where those colours are integral.
Dust and smudges: You know all about this, but just to remind. Forensically examine the image at 100%, section by section, side to side and up and down and clone out any that exist. If there are real things in the image that could be construed as dust, remove them. I know one guy who shot an antelope on light coloured ground on which stones were scattered, and it was rejected, much to his indignation. Birds in a sky are a famous example.