When I interviewed to enter the design program at the Ubisoft Campus, one of the interviewers asked me what, in my opinion, makes a good designer. And I remember the first thing I said was that the designer must play his own game. That obviously wasn't the only criteria, but to me, that's always been one of the, if not THE most important factors.
You shouldn't only play your own game because other games significantly broadens your horizons. But not playing your own game not only disconnects you from the game itself, but from the community as well. MMOs have a way of taking a life of their own because so many minds, so many playstyles come together and lead it in a path very often unexpected. But beyond that, what looks good on paper, what seems cool on your stand alone, what's pretty darn easy when you're running around in God mode (and even when you turn it off), is often a whole different story on a live server.
Reading comments from players is nothing like experimenting it yourself. Sometimes, you see players complaining about the way this or that was implemented and your initial reaction is to debate whether to pull out a violin or hand them a box of tissues. You go try it out with a regular account and go "yup, forget the tissues, let's send them some cheese and maybe even crackers" if you're in a generous mood. But other times, you try it and go holy cow!! Too many mobs, not enough mobs, mobs too hard, mobs ridiculously easy, drop rate insanely high, drop rate infuriatingly low, grief potential galore, etc.
There is nothing more revealing than walking into someone else's shoes even if only for a short while. That frustration, you lived it first hand. You understand it. I got pissed off (I mean cussing up a storm pissed off!!) fighting one of my own creations. And I thought to myself: "hey stupid, guess what you will do better next time?" And the thing is, what frustrated me the most isn't even what players have complained about on the forums. But in game, I experimented it first hand and witnessed players being aggravated as well. And if not for that, I never would have known the flaws of that design.
And that is the most invaluable information that no forum, book, article, poll or fancy pants theory can ever give you. You have to be there on the field and take that experience into consideration on your next project. It won't necessarily make you a good designer, but it sure as hell will help you at least be a better one.