It's my view that good content should seek to attract visitors, generate leads and turn them into profitable customers. I suggest that content comes in many formats (videos, pdfs, etc) and targets visitors based on where they are in the buyers journey. So you need content that educates, helps them decide and continues to add to their purchase experience.
I agree wholeheartedly with the thread above where the individual indicates that plumber general Joe has plenty to contribute to the web. He certainly has the expertise and subject know how, what Joe lacks is perhaps the understanding that if he fails to invest time and effort into creating content that educates, helps consumers decide and continues to add good value to his existing client base through his website, then Joe will never actually reap the rewards for doing so. Blogs and websites are not expensive, despite the comment above. Nor do I believe for one minute that Plumber Joe should not be trying to outrank Yahoo on how to fix a tap, that ship has long sailed. Plumber Joe would be better off educating his audience on how to spot a dodgy plumber, the questions to ask potential plumbers, what work should be carried out and typical costs etc. This is more likely to build trust, and trust will build customers. Plumber Joe may in fact be an expensive plumber, and because he has demonstrated his expertise and know how (through videos, how to guides, supplements etc), and he understands re-marketing, he gets the contract. (96% of visitors to your website will not buy on their first visit, so good content also persuades them to leave their details so you can continue to build that trust).
In fact, few websites really offer good content, because the attitude of most business owners is to copy and duplicate what already exists. Despite the vast size of the internet, I actually believe we are still very much in the infancy when it comes to good content. There is real opportunity for any person who has the genuine expertise in any given field, to produce good content and they will reap the rewards for doing so, not just on Google but via Social and Word of Mouth Marketing.
If any small business is struggling to understand what is good content or what good content looks like, it would do them no harm to have a good look at Hubspot. (I am in no way an affiliate or employee of them). They have some really good articles, posts, videos etc on how to create great content that attracts visitors and where they are in the buyers journey.