i have worked on sites that sold adult toys
i have worked on artist and photographer sites
i have worked on sites selling software
i have worked on general ecommerce sites
i have worked on sites that sell ads
i have worded on sites that just aim to change social policy
If you're going to do SEO, it often helps to have a clearly defined target market. For example, if you were doing SEO for real estate agents, you could create a real estate agent directory, and it would help you to help your clients. So there are benefits to specializing in a certain field.
if your aim is to be a long term full time seo it would be a good idea to create a couple niche specific directories in somewhat expensive fields that need work done.
it give you a huge advantage over other people.
doctors directory .com or perhaps yourrealestateagent.com or some jazz like that...