I read this so that I'd have it fresh in my mind before reading Glenn Beck's book with a similar title.
Paine carefully lays out the problems with a monarchy, argues in favor of separation from England, suggests an alternative government structure, and even offers up a strategy for building an maintaining a navy superior to that of England.
Paine makes so many good points that it's hard to believe he would use the few that were along the lines of: "if God wanted us to be ruled by England, he wouldn't have put us so far away from England." He also uses the "if you don't see it my way after my argument, you're just flat wrong" approach that I've never liked.
Published at a time when America had only one major debate ongoing, Paine (anonymously) weighed in decisively on one side, offered an immediate plan for moving forward, and donated his book royalties to the cause for which he argued. It's easy to see why this was one of the most well-received and influential publications in American History.