I have never heard anyone say, "The quality of Google's search results sure have improved over the past two years." In fact, I don't think most people have noticed anything other than the way that they have garbaged up local search results.
I think it was a good idea to go after the ultra-spammy sites, but those sites didn't show up in top positions very often. The problem is that there are too many good sites that have lost 50% or more of their traffic due to Google's shenanigans.
We know that Google does play politics. There are too many conservative sites that have always had a very difficult time developing Google traffic. I don't see Google as being much different than ABC, CBS, NBC, or MSNBC. Certain types of news are actively censored by these media outlets due to their pro-Obama agenda and it is moving the country to the left.
I think we would all like to hear your theory.