First off, when we refer to nostalgia or what I pointed out as "The Golden Age is 12", is not about "reliving when you were young", its more about "reliving the sense of wonder regarding that particular thing".
Nostalgia and "fun" are not mutually exclusive things. Nostalgia IS remembering the fun.
Nostalgia is not a bad thing, and even though Grogdog posted some reasons, I'm surprised at the negative reactions. I don't know about other people, but I don't use it as an epithet. There are games marketed towards the nostalgia market. All the "old school" PDFs that use Century Gothic font for the interior and try their best to recreate the look of TSR modules from 1977-1983, the ones that get Erol Otus to do the covers, etc. That's specifically being done for a reason.
I think the important thing to look at is just to be aware of nostalgia and how it can color or influence our opinions. The feeling itself is a tool, but it can be used for "good or evil", if you'll pardon the expression.
Hussar brings up a very good point. I think "the dark side" comes off when you try "too hard" to defend it. That gets back to the artistic points I made earlier. While art can be subjective, there are some standards for it that can be used to at least attain a rudimentary quality. Lets face it, much of the art in the 1e Monster Manual is not as well-drafted as the stuff that came in the later days of 1e, once TSR was able to get into bookstores and hire more people. The art was inconsistent because artists usually cost more than writers, and back then it was just people from the wargaming industry who moved there. And it goes double for layout and presentation--we have come a long way from the days of the xacto knife and paying for a printer--color, fonts, tools for layout, they've all improved and if people want to argue that the "old look was better" and try to argue it from an objective standpoint rather than at least considering it might be based on them experiencing it the "golden age", it could be a problem.
Nostalgia can also blind us after the fact, and get being thinking things were better back then, and can lead to myths about the past.
For D&D, I see a lot of old-schoolers complain about the changes after EGG left, but really, there are several holes in that theory. If the Blume/Williams war didn't happen, Gary might have still been spending his focus on Hollywood. Unearthed Arcana, which a lot of people criticize, would have influenced the revision, and the "hated Zeb Cook", who I think is on the brunt of criticism for being the "scab" who created second edition, would have been working with EGG along with Jeff Grubb on the 2e--just read one of Gary's final columns before the big battle occured. We would have likely seen Ed Greenwood in a creative capacity (as well as Roger Moore), maybe even seeing his Forgotten Realms and Bob Salvatore might have still be hired by TSR as a novelist and still created his characters--maybe Drizzt would have been on Oerth instead of Toril, for instance. So all this "bitching and moaning" about what might have been seems to be a more idealistic view of the past. I tend to think no matter what would have happened, you'd still have a segment of upset, grumpy gamers no matter what, simply because the rules changed.
Similarly, I think arguments can be tinged by nostalgia. On the one hand, I reject that there is a strict, formal, science to creating an RPG, and 1e works just as well as 2e, 3e, and 4e. The rules are the rules and they can be enjoyed by all, based on your tastes. On the other hand, there are arguments that can be made about how well-thought and versitle they are--"Percentile Strength" is a pretty blunt "add on" to an existing system, while stuff like 2e's THACO and 3e consolidation of Saving Throws into 3 distinct categories show a consolidation in an effort to make things easier to memorize as well as more versititily (1e saves are tied incredibly to magical effects, while 3e can handle new things such as radiation and science-fiction elements).
I guess what I'm saying is that people need to look at their own internal opinions and see if nostalgia might influence them. There is no guilt in just preferring AD&D to 4e if you enjoy it. I think most of the arguments come down to advocacy from either side, or fear that your opinion is in the minority, or dismissing the other perspectives as "stupid".