The final point is really important to me because I know that my interest tends to fluctuate. Every five years or so I feel like moving on to something new and leaving my main project. In the case of business, I want to ensure that there is a profitable exit strategy. The better you meet the previous criteria (profit margin, automated, scaleable and passive), the more money you can make when it is time to sell.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.