Got a passion? Helps if you have a real interest in an idea. I've found that first-round attempts I've made at sites have been successful (got a few sites that took very little time to set up, but now make $4k/mo for zero effort), but attempts to replicate that have failed (have a few more sites that barely cover their domain registration costs!). Without the passion, you would need to be
very disciplined, I suspect.
Here's something I just tried to see if it might help someone looking for a niche (since almost all are massively saturated, at least at first glance):
Hit Dmoz, control-click five sections on the front page. For each tab that's opened, choose a sub-sub-cat pretty quickly (maybe don't overthink your choices, but don't necessarily go for the largest option). If necessary, choose again on the next level. That should give you five things to look into further. I ended up with:
http://www.dmoz.org/Home/Family/Runaways/
http://www.dmoz.org/Reference/Knots/Fishing/
http://www.dmoz.org/Reference/Education/Instructional_Technology/Organizations/
http://www.dmoz.org/Health/Senior_Health/Fitness/
http://www.dmoz.org/Shopping/Food/Smoked/
Never know what any of these might turn up. Lack of decent, dedicated blogs, need for small widgets, simple calculators/tools, etc. With anything though, unless you have confidence in your content or idea, I would keep it quick and simple and not invest too heavily.