Constructive criticism can be very positive. Real Disney fans know what Disney is capable of, and we want to see Disney live up to their capabilities. Holding a company accountable to do its best work is in the best interest of everyone who loves Disney. If people don't speak up, write e-mails, visit guest relations, etc, when things are sub-par, the parks will not improve and the company will continue to skate on its reputation.
There is a distinct sub-set of the Disney fan community that takes any criticism (no matter how valid) and internalizes it, interpreting it as a personal attack. It doesn't make any sense but people have been doing it for as long as I've been part of the community. Do they feel like making criticism makes you disloyal to Disney Fandom?
My pet theory is that it stems from "fear of missing out" or fear of having missed out, that they missed out on the golden years of the theme parks in the 80s and 90s, that they missed out by not having been able to travel to the parks as often as others. This fear manifests itself in a knee jerk reaction where they have to justify that their vacations are just as good as any that took place in the past, therefore they are very sensitive to any insinuation that the current state of the parks isn't top notch.
I assure you, anyone who is on these boards is a fan of Disney, there is no way anyone would be wasting their time here if they didn't love the parks. Just because you love the parks and just because you vacation there regularly doesn't mean that you have an obligation to turn a blind eye when things are not up to Disney standards. In fact, I would argue the opposite, that if you are a real fan, that you are the only one who cares enough to press for positive changes. Disney fans deserve the excellence we know that the company can deliver, but we enthusiastically except less, that is exactly what we are going to get.