I like this article. Much of the advice on startups is biased from an investor perspective, this is something that's very real from a founder's perspective.

If I had to summarize and riff off of the ideas here, I'd phrase it as "Be very careful about where you're getting your information from." If you got your startup idea because you heard about something from a news article, you are way too late. If you got it from an Internet forum, there's a chance that you might be on to something, but it depends on how niche the forum is and how close to the bleeding ege. If you got it from a problem that you or a friend personally had and you know how to solve it right now, you may be on to something, but you have to carefully consider how many other people are like your friend and whether that's going to grow in the future. If you're in a niche community yourself and that community is growing rapidly, there are probably lots of things you can explore that might be fruitful startup ideas.