Well, it is great that few people shared their life stories about why they do microstock and photogrpahy in general,  but that wasn't the point of this topic.

To summarize the actual topic as stated above, these are some things, photogs new and old, just beginning or already sold their soul to microstock, can do to increase their acceptance percentage SPECIFICALLY at shutterstock: 

1. Bump the saturation and vibrance. They seem to love oversaturated images. I never got a rejection for 'overfiltering' as istock does often

2. Avoid generic architecture shots. Architectural details seem to fair more than generic architecture.

3. 'Uneven lighting' usually means white balance is not percieved as being correct.  Fixing white balance in PS helps, but I still haven't solved this satisfactorily.

4. If you cannot find any artifacts after close examination at 100% of your image, most probably it was rejected for not being percieve 'stock worthy', artifacts are often used as umbrella, and you can spend hours looking for non-existing artifacts.

5.  Make sure your model release is upright.  They do reject images if the model release is sideways.   Undecided

6.  Be prepared to have your more creative shots rejected.  Examples: deliberate motion blur for focus problems, on-camera colored filters for white balance or too much noise reduction, silhouettes for lighting, high key shots for over or under exposure.

7. Generic nature pics are subject of very close examining whereas industrial shots criteria are much less strict

8. Downres the images to 6MP or 4MP (you get sharpness and reduce noise, SS pays the same for any size)

9. Submit in small batches. If you have 50 images to submit you will normally stand a better chance to get them accepted if you submit them in 5 batches of 10 (waiting till each batch is reviewed before you submit the next one)

10.  Example workflow for reducing noise rejections at SS
   1.) noise filter with NeatImage, using autoprofile
   2.) clean the rest of "noise" (eg skies) with blur tool - 65%
   3.) oversaturate
   4.) resize to 4MP
   5.) add IPTC data
   6.) upload via ftp


cheers,

I rarely get rejections on shutterstock. As others have said before, its not a matter of getting your pictures accepted, its finding what will sell. Take the rejections you get and look at them carefully.
I look back at some of the stuff I submitted two years ago and cringe now. The rejections I got back then were one of the best tools for improving my skills. Your family and friends will tell you that everything you shoot is great.  Shutterstock is going to tell you what you need to hear.
Pick random keywords in a word search and see what comes up by most popular. That will show you not only what is a good image, but also what sells.

There is a great thread going in the SS archives somewhere about the life of a reviewer worth checking out.

My acceptance/rejection thing has really changed, thanks in part to this thread and other things.  Much of it lies with "will this photo sell, and would we be proud to sell this photo?"  If either of these answers is a no, expect a rejection.  Few sites are nice like BigStock or IS and really tell you what the deal is, typically you get a canned response and that is the end of it, figure it out on your own. 

It is either A: head back to the drawing board with the concept, it won't sell, or B: Good concept, but poor execution prevents us from tarnishing our good name by selling this crap to our customers.  I wish that all rejections came in one of these two flavors, that is it, but alas, we get the myriad reason that half the time or more aren't really why they rejected them, just an excuse. 

SS rejects all of my weaker shots for focus, I thought my lenses were out of whack or sharpening was an issue but no, the just didn't think that they would sell so hit the focus button.  I've never had a focus rejection elsewhere, BigStock told me that the DOF was too shallow once, cool, different than focus though, it's kinda grating when they tell you that you can't focus your darn camera, pretty insulting, but I let it slide as "we don't like it", which is fine, their sell through rate is too good to question their eye for salablilty.