Most of us have skills enabling the ability to rank in web sites/pages for search terms in Google. For [I assume] most of us here, that is our main skill.
How we monetize on this skill varies widely amongst us, in fact it probably varies regularly for ourselves also.
I got into this "trade" by helping my father work out how to make and promote a website for his company whilst at university. Doing that gained me certain experiences and expertise of which I wanted to work on, so my first SEO career commenced with consultancy, a very high paying job for a university student.
From there I worked out that I was creating enormous value for these customers, getting them rankings for very competitive terms, and charging them relatively negligible fees.
So I started my own sites, but my skill was ranking for search terms, so I simply produced affiliate sites. Being content with the easy income, I gave little thought to a long-term strategy. My sites would get banned, but it was easy enough to just start new ones. The content was weak, and I guess you would call them doorway sites. Then Florida hit, and this strategy was gone.
From there I was devastated I had lost my gold mine, so delved in a bit of black hat, as I really wanted to get my income up to pay for my house I was now building. Again, of course these sites would get banned continually, so I had enough of having to restart again and again, and was looking for a more sustainable business model.
So I would start sites with real content, but again I had little skill in copy writing, and wouldnt call myself an authority on many subjects.
From there making real web sites, to actually help people, opposed to just put money in my pocket, and having a sustainable business model I have experimented relentlessly, with different industries, different specialties, investing in content here, development there, etc. Until now I have reached the stage where I think I actually get it. Sure, this is partly brought on by Panda, but it has only really reinforced what I already knew.
So now, I hire proper copywriters, who can not only write good content, but are also very familiar with the subject. I think wikipedia is a good guideline. If you are not offering your visitors anything more than that, then you haven't got it. And from here you reach your sustainable business model.
Nowadays SEO comes to us so naturally, we don't bother tweaking keyword densities, count, and other redundant optimization techniques. We can't link exchange so easily, and link purchasing can be costly. So the evolution of SEO, is offering you visitor the best user experience you can come up with.