I've never ridden M:S, and don't really care to. I've said from Day One that I think it's way too intense for Disney (or anyone). There's a reason they train astronauts before they launch into space. Kennedy Space Center's only a few hours away if you're set on that. Disney could have come up with a much better themed thrill ride. Anyways...
Everybody wants to blame either Disney or the parenting. I don't really think there's anyone to blame here. It's just a horrible tragedy. The parents may not have known about any pre-existing medical problems. They also may well not have known what kind of ride Mission:Space is. True, there are a lot of warnings in front of it. Those same warnings can be found outside Big Thunder and just about every mild-to-wild attraction at Disney. There are more (and bigger) warnings at M:S, and it seems like you'd at least ask a CM just how bad it is, but that's what height restrictions are for, I suppose. The height restriction definitely needs to be raised quite a bit for M:S. I expect it will be.
I don't think Disney was wrong to open it back up the next day. There was an investigation. The attraction was not malfunctioning and there is no evidence that the death and the ride were in any way related. On those grounds, why keep it closed and disappoint thousands and thousands of eager, paying guests who want to try it out?
Disney can't do age restrictions on their attractions because kids don't have ID cards. Cast members can't tell the difference between a 6 year old and a 8 year old in many cases, and they shouldn't be in the business of enforcing guestimations. Also, as T/P Fan pointed out, size is really the determinant factor for everything but fear.
Renata is exactly right about the reason that Primeval Whirl has a high height restriction... the nature of the safety bars (instead of the buckles you find in most Disney rides). That ride is also a lot less tame than it looks from the ground.
I don't think it's fair to say that "loads" of people die or are injured at Disney these days. True, there's been I think 2 guest deaths that were the fault of a cast member/ride (and 2 cast member deaths, I think), but most of the injuries and deaths are the result of disobedience, ignorance, and stupidity. 4 deaths are 4 too many, but then I don't think it's entirely unsurprising over a 50 year period. It is very sad, though. Difficult to think about. Whether it was negligence or just an accident is probably debatable.
Mr. Toad is right that maintenance was suffering at the Disney parks for a while, but that seems to really be turning around at both DL and WDW lately.
IndianaJP is right that Body Wars and the entire Wonders of Life pavilion now run seasonally. It's currently closed but will likely open for the summer soon. There's something going on over there now, so maybe it will return to full-time operation one day. In my opinion, Body Wars should close and stay closed (now I have a ride for the "Overrated" thread!).
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-Aaron