I have written before (and here) that I believe that if you have a site that is over optimized for search engines chances are that your human visitors could find it hard to read, use and understand.
I ask my customers what good is having all of the traffic in the world if they just stop by for a second and then click to go somewhere else? Search engines are now thinking like this. They can tell how long visitors are on your site and how many pages they are visiting and I think this is incorporated into their algorithms.
My advice is to read your content out loud. If there’s someone else there to hear it so much the better, but even if you are by yourself it still works. If your content sounds like it’s stuffed with keywords or sounds unnatural and forced it’s probably not good for your site.
For SEO there are rules for exactly how many keywords to use and where to put them, but this formula (besides being a search engine secret) is changing all of the time. That means to keep up the ranking that this has achieved your content may have to be changed often.
If you write in a natural sounding way it will be a better experience for your human visitor and that really is what the search engines want to provide for their customers (the searchers). That also means that your content can stay the same and not be affected as much or at all by the changing search engine algorithms.
There are some good points in an article by By Rusty Ford on the SEO-News site called “Key Wording Your Web Site With Natural Language” that may help you decide what is “Natural Language.”