I think this song is talking about revolution.

Feel Good Inc. could refer to a massive corporate entity, in which the people in charge only think about materialistic things, like having a million women, or being the ruler of everybody else. However, they are so caught up inside their own tower, that they are completely miserable despite all of the things they have. Ironically, these same people seem to enjoy talking about how much they are enjoying life in an attempt to make themselves appear all the more powerful and superior.

Still, there are lines where the person(s) talking seem to look down upon the "lower" people in distaste, saying that their talentless, carefree nature is the cause of all of Feel Good Inc.'s misery and boredom: "They've just gotta go cuz' they don't know whack" and mentioning them being "damn-ass free" After that, the person talking goes on to talk about being in a "melancholy town that never smiles" and not being able to sleep.

Then the next thing you know, the windmill flies by with the sun rising in the background. Also, there are helicopters chasing after it; these helicopters seem like they're part of a super-modern world and are trying to stop or capture the windmill. I, personally, think that this windmill represents a new thought, or revolution. One that allows happiness outside of materialism and power; outside of "Feel Good Inc."

So in the end, I think Feel Good Inc. represents an idealized "perfect" world set upon us by powerful corporate and government people, whereas the windmill represents a new wave of thought that doesn't rely on objects or social status for happiness and fulfillment. Some clues of this are in the chorus: "Windmill, windmill, for the land".

I'm thinking that the guys who write the songs for Gorillaz thinks that it's time for a revolution before we end up living in a totalitarian government. Then again, they could just be talking about the band's status... who knows?