While I agree with you on the idea that the criticism will ultimately close down discussion and paralyse more enlightened debate about the qualities or possible improvements to another game, I think there is a reason behind the use of the phrase that you didn't touch on.
A lot of games, like Guild Wars, strive to do something completely different with the genre than with WoW. Sometimes the "go back to WoW" statement is more of a summary of an opinion that, well, the proposed feature or suggestion would only make the game more like WoW rather than necessarily better. It might even be seen as a move away from the purpose of said game. People summarise their opinions in such a way often. It's just a part of interacting on the Internet.
Let me be more specific. If, say, someone were to propose that Guild Wars should have a level cap in the eighties, this would ignore the philosophy behind the game; that is, the level cap is much lower than in other MMOs because this fosters stronger balance and more skill-based gameplay in PvP matches. The simpler a system is, the more likely it is for the developers to create something fair that can only be exploited for so long. In such a situation, I think the proposer is making the massive assumption that the WoW-style approach is the only way to go in an MMO. This is obviously the wrong idea, and sometimes people rightly get impatient with this.
You could say that I understand both sides here. At the end of the day, WoW is great, but it's not a perfect example of how to do everything right, and other games should not follow it blindly, an idea that doesn't always occur to fans of WoW who then try other MMOs. By the same hand, it is unfair not to explain to such people what they're missing in a civil and effective way.
It's a good thing that developers seem to be better at this than members of the forum have, so fair and balanced criticism does get to the people that count regardless. I remember Gaile Gray, a major developing figure for GW on their official forum, had to deal with numerous forum dramas and complaints about nerfing and low level caps, and she always did so in a way that I respect. I'm sure that Blizzard's teams are like this as well.
