I am someone who knows first-hand what it can be like to deal with a child with Autism having a meltdown. But I am also someone who has no experience with the GAC because we are only attempting our first trip with our son this coming December. I do not know what these changes truly mean because I have nothing to compare it against. I have been planning our trip for the last 3 years... and I have been slowly preparing our son, building up his skills and coping mechanisms to a point where we think he can handle the parks as much as your typical 7 yr old boy. But we don't know. Autism could put the monkeywrench in our well laid plans, so we are staying flexible and open-minded and if we need to use the new system, we will do so.
Currence nailed it pretty well, but there are a few other things that could have some of these parents in a flap. The sequencing is just a small part, the waiting, another part, the sights/sound/smells are yet another part, and so on. Depending on the child, any one of these things on their own might be bearable, but for some Autistic children/teens, all of these things together present a real challenge. Where the parents are most likely concerned is that IF/WHEN that child hits their trigger or breakpoint, the meltdown can be spectacular, not in a 3yr old tantrum way either. Depending on the severity of the Autism, and the overload of sensory stimuli, the kid in question can do any or all of the following : scream, cry, rock back and forth, flap their hands and arms, try to flee, try to push past whatever obstacle is in their way... some could even be so overloaded they faint or vomit. Now these are extreme, I grant you... but isn't it anyone's nightmare to be stuck in a long, slow-moving, potentially claustrophobic queue when your body is screaming for something (a bathroom break, nausea or what have you)? Imagine being stuck in the tiny DL Peter Pan queue when this takes place and you get some idea of the fears some of these parents have. Likewise with the back-and-forth between attractions and kiosks... that is definitely something that may cause a lot of problems for these families. Little Toby may well freak out when they have to return to the kiosk and get another card for another attraction. Suppose he really wants Jungle Cruise and ONLY Jungle Cruise and the lineup is 90 minutes (worst-case scenario), so Mom and Dad need to make a decision on whether the wait is doable, or is there something else shorter that they can get a card for. Or imagine the indecisive child... the family ahead of yours in the line at the kiosk is taking forever to either chose a ride or is waffling back and forth, or even trying to convince little Toby that Jungle Cruise is too long of a wait. How long can you stand there waiting for your turn at the kiosk... and then your child breaks down. What if the line at one of the 4 kiosks is 15 families long...I can definitely see why parents are upset.
That being said, Autism is a spectrum disorder and covers a huge variety of individual issues. NO POLICY no matter how well thought-out is going to be able to cover all the bases. Disneyland and places like it are for everyone to enjoy. I think that Disney has always gone above and beyond to accommodate people, which is why they are the most popular family destination bar none. BUT with the rising numbers of people with accommodation needs PLUS the people who cheat and abuse the system, we have a tempest in a teacup. I really applaud that Disney is not only acknowledging that a system is no longer functional, and then actively taking steps to mediate the problem and consulting with disability groups to do their best to provide EQUAL (not preferred) access to its attractions.
The bottom line for me is this. I want my Autistic kid to be able to enjoy Disneyland every bit as much as his typical sister. I want our family to have a magical vacation like any other family. BUT I know that it is not a requirement that Disney bend over backward to accommodate my families needs. It is up to us to do what we can to help our son cope with the waiting, the lines, the noises etc. We took him to a local small-scale amusement park to see how he handled the rides themselves (no problem, surprisingly), as well as waiting in line (again, he did surprisingly well). In fact the only true meltdown moment he had was when he thought he was going on one ride that he wasn't tall enough for... we diverted to another ride that he had just enjoyed, but the freakout ensued and we had to leave the ridezone immediately. We got him calmed and carried on with our adventure. Sure I got nasty looks and overheard a few comments, but we did the best we could. I found out much later that we could have gotten something like the GAC at that park, but I don't think we actually needed it. I hope we won't need the DAS when we get to Disney, but we won't know till we get there.
So that's my 25 cents, as well as a bit more info on what an Autism meltdown can look like.