As someone that's new to stock and kind of new to photography, i'd place stock photography at the higher end of the skill level.
Why do I say this? Because you need to be technically perfect at everything. Subject matter, exposure, composition, recent events, upcoming events, post processing and everything else that goes into a professional looking image.
i've seen videos, i've read blogs, and bought ebooks, and they all say "anyone can get into stock photography, all you need is a camera and good lighting" I'm sorry but that throw away ideal is just simply not right.
I think you need to understand all the principles of photography, how light effects a subject, and how to create an image that tells a story, conveys a message or provokes thought. If you think that buying a camera and taking pictures is going to be enough then it's not.
I'm on Alamy, Shutterstock, Fotolia, Bigstock, Can Stock Photo and have failed the iStock submission twice, I can tell any new person getting into it, to be prepared for those images that all your friends say is perfect, to be shot down in flames on these places. It really opens your eyes, and takes you down from that higher place your peers, family and friends put you on.
It's been the most stressful journey in photography i've ever been through and if you don't have proper lighting, and you're images are borderline with technical detail then you're going to struggle, which in turn will really deflate your confidence.
On the flipside, if you are determined, and can take rejection with a pinch of salt, then go for it. If you have the time to gather thousands of images, and I mean you're going to need thousands of images, then go for it. However, if you are the type of photographer, that wants to make some pocket money from a few hundred images that meet their submission criteria, you might find your images (unless they are totally different to what's on there) drowning in a sea of the same old stuff.
Don't go on and see what's there, that's the worst thing you can do. You end up trying to emulate what sells well. Take the best pictures of what you do best, and as long as it tells a story, invokes a thought process in people, and you'd buy it, then you're on to a winner.
I'll probably start a fire with this one, but based on my experience over the last 3 months, this is the conclusion i've come to.
Stock photography wesbites demand the best, and remember it's photographers at their peak level that review these images, and if they see that you took no effort to snap the pic, then they won't even look at it twice. I've tested this deliberately and it's quite amazing how some images get accepted by some and not others. There's no pattern to it.
The harder the image is to get, the more it will sell. Just my observations 