I think one reason why a lot of people are hesitant to charge money is that it completely alters the relationship with your users. Paying customers deserve a much higher level of service than free users.<p>This is probably just another good reason to charge though. Taking other people's money is a commitment to providing service to them in exchange. You can't have too many good reasons not to give up.
Making money at a startup and having fun are (thankfully) not mutually exclusive.<p>One could argue that the more revenues you get, the more fun a startup is.
"Business isn’t a game, and I think some web entrepreneurs treat it that way."<p>I like to think of it as a game, a fun problem to explore. Helps to keep thinking fresh and flexible. Just a matter of defining appropriate conditions of success and failure (which may itself be a part of the game ;).