I just know one thing. About 18 months ago I started reading the google forum and following everyone's advice and optimizing so that I would rank well in google. At the time I was doing very well in yahoo and msn. My visitors were few.
After all that I did my site ranks very well in google...extremely well for some terms and I now am nowhere in Y or msn. I just checked my stats and have almost no visits from msn and Y doesn't seem to know I exist. However, my total visitors are up 250%. All from google.
Basically what I did was remove everything that resembled spammy or blatant seo and went for more content. What a good site should be.
The only reason I stopped into this forum was because I was curious if something had happened in Y. I have no visitors today from Y. But no earth shaking things. So..I checked how I was ranking. zilch. nada. I am non-existent.
But I wouldn't trade back.