One should think about their skills, hobbies and basic daily habits as well. Do you like to cook? Be a personal cook or dinner parties. Baking? Make wedding cakes or cupcakes for weddings/special events. Do you enjoy cleaning? Trying going to banks/repo companies and ask if they are in need of a repo cleaner or market yourself as a all-around cleaner. Know how to mow a lawn? Get paid to do it in your neighborhood. Sell products from large companies – Jewelry in a candle, Sentsy, Health and home products – etc.
Dependency: This is the biggest con, so pay attention! Affiliate marketers depend on the merchant to make their money. If there were no merchants or affiliate programs, the possibility of being an affiliate marketer would not exist. It even becomes more frustrating if for one reason or the other a merchant suddenly stops running the business or their affiliate program. This can potentially deprive the affiliate marketer of his one-time steady source of income.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.