The ONE common feature I find among my favorite blogs and websites is that the creator is really knowledgeable and passionate about the subject being promoted on it. It seems everyday, IMers suggest that you go into Google Keyword Planner and find the niche with the most searches or is underserved by blogs and make a killing creating a website, writing some (usually mediocre articles) and promoting affiliate products or ads on it. Well, I've tried that and it hardly works unless you can get your self totally interested in the niche (or else you're gonna give up after writing 20 1000+ word articles on the subject and have run out of ideas while getting only 300 visitors a day.)
So in other words, if you've been a trucker for a decade, then try the trucking niche. If you have a PhD in Chemistry, then Chemistry is your niche. If you're a Lawyer by day and an Anime otaku for many years in your free time, you can go with either the Law or the Anime niche.
Forget discovering what niche other people are following. Go with the one you're most passionate and knowledgeable about.