You might be right. I'm not familiar with other agencies, so i probably should choose my words with caution.
Please note that my only intention here is to address possible reasons to the high rejection rate at istock. Since this seem to be a big issue here, you know...   

And when we're talking about quality/rejections...

It would be easy for istock to make a profit from selling pictures of lousy (or not optimal) quality - The previews aren't very big and doesn't say much about the quality, so the customers will have to trust the agency in order to buy images.

It is not difficult to get exclusive with this limit. If you provide good, useful pictures, that is. (EDIT: I have just a little over a hundred pictures in my portfolio. Very few, if any of these are of the classic stock-type.)
There is way too much whining about low acceptance rates. Hell yeah, I'm facing rejections all the time just like the rest of you, but most often I learn something from it.

Upload limit: Personally I think it is a good thing, that they decide to concentrate on the already long cue. I guess they are just as interested as the contributors to have a large collection. As some of you might know, Istockphoto actually makes money on selling pictures - Of course they want a big collection! If they feel they are short of inspectors, I'm sure they'll put some new inspectors to training.