I'm looking to start up a new site with the aim of earning an income from it and so looking for a niche. I was thinking it would be good to get a list of things to look for when examing various markets online and spotting an opportunity. Anyone got some good tips? Here are a couple of mine:

Do the most highly searched for keywords in that niche searched for enough?
Are sites on page 1 of google optimising it in their page title?