I recently added a new post to my blog (twcdm.blogspot.com) talking about what kind of images you might want to avoid uploading/selling.
twcdm.blogspot.com is a blog I started to share tips, tricks and tutorials about photography, Photoshop and getting into stock photography.
A lot of what is on this list is only on the list because there is so much of it out there already, not because they are low interest subjects.
*Disclaimer: I put this list together based on my own experience, just because something is on this list in no way means it wont sell.
Photos taken at the zoo - Taking photos at the zoo can be great practice, but unless you can make the shot look like it was taken in the wild these have low sales potential.
Flowers - Unless they are magazine quality with the name of the flower don't even bother.
Clouds/Skies - Everybody takes cloud photos. I actually take them to use later if I need to replace a dull gray sky in a landscape or something. Unless they are really good they are not worth the time to submit.
Sunsets/Sunrises - This is another one that there are nothing wrong with them, they just have to be really good to stand out against all the ones that are for sale already. Make sure those horizon lines are straight if you are going to submit them!
Snapshots - Snapshot is a broad term basically meaning unprofessional looking or unappealing. If it looks like a five year old could have taken it, its probably in this category.
Photos with poor composition - Cropping or cutting off part of your subject. If your taking a portrait try to keep all of the models head in the frame. When someone buys your photo they can always crop it down, but they cant "uncrop" (think I just made up a word).
Under or overexposed photos - Try to avoid taking photos where there are blown out areas or the overall photo is too dark. Keep an eye on your light meter and histogram on your camera when shooting.
The rest of this blog can be found here.
For the record I am guilty of submitting and selling photos of all these things. I'm not proud of it, but I have learned a lot by getting it wrong the first time. Also there are always exceptions to the rule. What I'm talking about here is really knowing what your doing and doing it wrong on purpose for artistic effect, not doing it wrong on accident.
Hope this helps!