OK, this is my first post and I don't make my living shooting stock, so take this with a grain of salt. But, I do have a lot of experience working at agencies.

Study the sites on the right. Many have useful blog articles that discuss trends, highlight what's selling. Some sites show # of downloads, views, popularity. Study it. The agencies want to make money, and it's in their interest to educate you so you create photos that make them money. They make money, you make money.

This forum has some very good perspectives, but in my experience it seems like it's used mainly to discuss the pros and cons of individual agencies (royalty rates, reviewer standards, sales overviews). There seems to be less focus on guidance about how to carve a niche and create good content. That's not a bad thing (obviously users here aren't going to divulge all their secrets), but keep it in mind.