Scott,
There may be occassions when one ought to have played a course before offering certain types of opinions, but when it comes to general rules about the qualifications or experience one needs in order to qualified to offer an opinion, I tend to agree with George Pazin. Usually, when someone is called out for commenting without personal experience it is by one who is wielding his own personal experience as if it were some sort of ultimate trump card, like personal playing experience made one's views obviously superior all matters even loosely relating to the course in question.
In other words, the "lack of personal experience" accusation is often made when someone disagrees with something but is unwilling or unable to provide a reasonable refutation.
There are plenty of sources of information, and personal experience is only one. So long as the poster is willing to explain the reasoning and source information behind the opinion, any sort of opinion is potentially valuable. If not it should be easily refuteable by resorting to the merits rather than by simply claiming that the one with experience knows better.
Moreover, while personal experience is obviously a valuable source of information, it also happens to be the most immediately susceptible to personal bias and institututional prejudice. People love their clubs, and their own emotional attachment impacts their objectivity.
Don't believe me? I was once berated for weeks because I had the nerve to challenge the actual distance of a certain golf hole (and the distance of some famous drives) on an exclusive American golf course. And when I say berated, I mean berated-- called a liar, an idiot, a fraud, hassled off-line, etc. One poster even stormed off the site in a huff, insisiting he would never discuss anything with me again! All this because, in their opinion, I had no businness offering an observation about a course I had only played once. Never mind that I had repeatedly explained my methodology and it was never seriously challenged; those who thought they knew better weren't going to allow their legends to be questioned by an outsider.
In my opinion and experience, this sort of thing is very common around here and poses a much more serious challenge to the usefullness of this website that those who offer an occasional opinion without having played a course.