Reading through some excellent blogs and randomly following links I came across a post discussing if SEO was a service or a product. In fact it wasn't really asking the question so much as stating that if you view SEO as a product you will more often than not be disappointed.
It struck a chord with me.
For example, there are many link building services offering x thousand links for y pounds (dollars, whatever). This is a very simple transaction to understand. You pay me some money, in return you receive some links. That's SEO as a product. What may not be understood is the effect, if any, these links will have and if they do have an impact how long will it last. But that aside, people 'get' a transaction like this.
What people struggle with is the, SEO as a service model. You pay me x pounds and I will try to improve your rankings. People often ask us, how much will it cost to get me to the top of Google for some keyword or other. They simply can't come to terms with the fact that organic SEO isn't an exact science.
What an SEO can do is provide examples of sites they have worked on that are ranking well, but these don't guarantee success in another domain. An ethical SEO still can't say, x pounds will have the desired result.