If you compare your photo of "subject X" with those already online, and the online ones are significantly better, then even if your photo were to be accepted it probably would not sell so well. Bear this in mind when a photo is rejected.
Images are frequently rejected by microstock [2] sites (especially if istockphoto were deleting all the non exclusive images in the queue so they could make one if their landmark images be contributed by a precious exclusive member) but having an image rejected should not make you feel rejected!
The Reasons
These are the most popular rejection culprits, some of which you can fix easily:
White balance, It's to be said I have never had this as a rejection problem, I don't know why (the recent purchase of a factory calibrated display probably helps), microstock images tend to be bright and bold anyway, and I just use auto white balance or film where I have no choice and that's it. Lazy. I did have a series of photos which I had lit with brightly coloured lights (the sort of effect that now looks a little dated, I admit - photodisc from c.1995 I think) these were rejected as "perhaps use different lighting" lesson learned. Colourbalance / Whitebalance(if you are having a problem) should be set in camera with a white or grey card before you start to shoot.
Focus, and inappropriate depth of field. This one is subjective, but the main subject must, must be in the focus zone. As a rule use a narrower aperture to keep more of the image in focus, but not shut down all the way as you will lose sharpness overall with many lenses if you do that. You should find out about your lenses or cameras 'sweet spot' often F8 is a good starting point for an SLR lens. More about Focus related Rejections and Depth of Field [3]
Over/under exposure (the bane of my life), It can kill off any creativity you might have when it comes to making moody dark images or bright shiny ones. Often microstock reviewers just won't like them, so I tend to submit a conventionally exposed somewhat flat looking image, and if it suits do something creative high key or low key with the metering and submit a separate shot and let the agency decide the one they like. Indeed, some will happily reject both because they are "under or over exposed" leaving you wondering which it was what.
ISO [4] or Noise [5] or Blur - set your camera on the lowest ISO it has and use a tripod. Simple. You could also try downsizing your images before upload, noise is reduced as you reduce the size of an image (but of course you will no longer be able to sell high resolution versions, which are more valuable) read more in Noise and Artifact image rejection [6]
Composition - This is such an abstracted reason for failure, but often the reason is not conventional rule of thirds composition problems, often something like the fact that there is a distracting background or something cluttering the photos that is not adding to the 'story'. As a typical example, if you were to use your own home as a location, then you would need to remove all the clutter and day to day objects there, leaving the space open and clean. Model microstock office workers are frequently pictured in strangely empty office surroundings, desks without pens, telephones or desk lamps, walls without pictures etc, or people depicted floating in mid-air on a white backdrop.
What else might be Wrong?
If you submit work to several sites then it's likely that at least one of them will provide you with good feedback. Don't worry too much about rejections from sites that describe "rejection might be due to: and then list every possible reason under the sun" see what the other sites say. If there is even the vaguest amount of consistency about it then try to make corrections based on that. Many of the reasons will be easily corrected next time, like wrong categories or keywords etc, if you take due care when submitting you should never get any rejections for these reasons, especially once you have experience and know how to 'play the game'.
If you have a high number of rejections for noise and artifcats then check to see if your camera has a dirty sensor [7].
What to Do...
Rejections tend to follow two patterns, consistent (simple to fix) and rejections for random or bizarre reasons (much harder to understand).
I guess I have been in the microstock game for too long, I rarely get images rejected for consistent reasons, and those that do I often suspected will be rejected before I start, usually for grain or noise that I was hoping I could 'get away with'. If your images have been rejected for consistent reasons on all or many sites, review then reasons and act on them. Many sites suggest courses of action to take, It should be clear where you are going wrong, the list above will help with the main reasons beginners face.
Most of my rejections are for seemingly random unfathomable reasons and only from one or two sites, in many cases it's just best to accept it and move on.
I have come across my images being 'out of focus' on one site and 'over sharpened on another', go figure? It usually means that the image was not all that great in the first place, and the reviewer just didn't really think it would sell as microstock so they clicked reject. On reflection these images often don't compare so well to those already online covering a similar subject.
Every photographer will tell you stories about bizarre laughable rejections, I once had a shot of a Venus fly trap, accepted at Alamy and every other microstock site but one, their rejection reason was that the photo was 'out of focus' now as it was a fairly narrow depth of field image the foreground out of focus and the background out of focus with just the centre where the plant was in focus, it stands to reason that some part of the image (in this case the subject) MUST be in focus, so, well, laugh at the reviewers stupidity and upload some more.
I'm not suggesting going blindly on here, if you spot a pattern over many images then you can act on it, just don't lose sleep over image rejections, life it too short.