It's too long for me to quote here, so follow the link and read it. He makes a few specific points I'd like to touch on:
In GC's second post in the thread, he talks about why they don't balance around population, and how it's OK to not have all 10 classes at a perfect 10% balance.
I want to be clear about this: I agree with him that there is no need to strive for exact balance. There are devoted fans of every class and spec. They all appeal to some people. Just because fewer people are attracted to demonic wizards than they are to shining holy knights is no reason to simply nerf Paladins until their population is even with warlocks.
I also agree that simply nerfing a class to balance population is yucky.
But, I do not agree with his entire post. For one thing, simple aesthetic preference, or even game mechanic preference, are only responsible for part of a class's population. There are always people flocking to classes perceived to be overpowered, and avoiding a class they love because it seems underpowered. Acknowledging personal preference does not excuse you from having to consider population in your MMO design.
Also, he's thinking too small, just in terms of obvious numeric buffs or nerfs. To borrow GC's own phrase: there are more levers you can pull.
How about the races? Blizzard has already taken good steps in this direction by not allowing either of the new races to be Paladins, and allowing the new races to feature most of the less popular choices. But why can't Night Elves be Shaman?
How about group buffs? Being a hybrid is too desirable at the moment. Let them keep the current advantage of role switching without also piling on the legacy of buff variety that you held over from TBC in the name of not rocking the boat too much. Give BoK to rogues. Have Ret Paladins only give one bonus buff instead of 3 (yes, by simply being there and specced Ret, you give 3% crit, 3% haste, and 3% damage on top of replenishment, a blessing, and the aura's base effect. I'm not kidding. Currently, a rogue can bring, at max, ONE buff to a raid.) and spread some of them out to pure classes a bit more. Inviting a Paladin to your 10-man should be a trade-off: flexibility for utility. Balance it so on his own, a Ret Paladin does damage close to the rogue, and can switch roles if needed - but the rogue brings 5 buffs and the paladin only 2. So every Paladin you invite to your 10-man raid gives you more reason to add a pure class to make up for the buff deficit. Right now it's the other way around, exacerbating the population problem.
How about ease of play? Blizzard is pretty on-the-ball about balancing the potential dps numbers each class can reach. But they haven't done much lately to balance the ease of playing each class. It sounds like they may make strides in this direction in Cataclysm, especially by forcing Paladins to have a rotation that matches the others more, and by giving DKs, Rogues, Hunters, and Warriors more user-friendly resource systems, and simplifying the Feral Druid rotation. Make Paladins apply an array of debuffs while reducing the prerequisites and build-up time of "pure" dps specs, while leaving all specs with similar mathematical damage potential. Make Paladins and Druids have to wrangle their abilities carefully and thoughtfully to hold equal aoe threat to a Warrior or Dk who isn't even looking at the screen (ie reverse the current situation). All 4 classes should have the same threat potential, but it should be easier to reach that potential for the less flexible class, instead of the other way around.
Some example changes: Crusader Strike does less damage, but applies a debuff that increases the Paladin's holy damage. Divine Storm is no longer cost-effective on single-targets. Holy Shock becomes a go-to nuke on a longish cooldown, but Exorcism does more damage but requires an Art of War proc, which is harder to get. Judgements do more damage to enemies with the Righteous Vengeance dot ticking on them, so it makes sense to plan when you use it. Tanking Paladins find that Consecrate has a longer cooldown than its duration, meaning you -OMG - actually have to think about when to use it, god forbid!. Meanwhile, rogues spam SS and SnD/Evis.
OK, went off on a bit of a tangent there. Back to GC's post:
At one point, GC objects to a player who suggests that the classes at the top "should be made miserable" I object to that idea too! I agree that the goal is to make sure everyone has fun. But the problems I see with GC's stance are:
1) It's not a dichotomy. You can make moves to help balance class representation without making anyone miserable. In fact, the net result of such changes can actually be to increase overall happiness! I think it's disingenuous to suggest that population should be discounted as a balance concern because you don't want to make people sad.
2) Representation differences can often mean there is a real balance problem, or a dangerous perception problem. Either way, it may need to be addressed in a way that makes some players have less fun, temporarily, in order to make the game better and more fun overall.
He seems well aware of perception and population as issues, but I'm not sure he takes them as seriously as they warrant.
I'd like to close with the best and most perceptive GC quote of the whole thread:
"We know (and so we have to be very careful about it) that players will do monumentally unfun or even self-destructive things if it conveys even a small advantage."
Yet another reason why the developer's caretaking of the playerbase is so important, and why population balance should be a bigger consideration.