Complete lack of any AI for the monsters

[Walkiry]

You cannot have a system where anyone can play whatever role they choose and have success without either making the encounters trivially easy for anyone employing the classes in their correct roles, or making all classes equally good at everything, thus obliterating the need for distinct classes or spells or abilities at all.
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However, for those 40-man raids, any responsible designer will ensure that there is a role to be filled by each of the eight classes; it is not outrageous to expect that members of all classes will be available and that each will want to be useful and effective.  Asking for a 40-man raid missing a class or having classes playing off-roles to be as effective as a 40-man raid where all classes are present and working to the best of their ability is simply paradoxical - you cannot design a game that way.
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You're going way too far. No, I don't expect, want, or think that a group with many off-role players should be as effective as a perfectly geared group, but I would like them to be able to overcome challenges. To do that you have to give them more options to overcome those challenges.

Going back to our mail wearer tanking, I don't expect him to tank as well as a decked out defensive warrior; most likely, one'd need much more healing, or a larger stack of debuffs on the enemies. You could say that a mail-tank would need a different group composition to be successful. This mail wearer could, perhaps, need a more defensive "spec" and thus lower the other capabilities (most likely through itemization, as that's Blizzard's choice of poison). And you can do this pretty well without making encounters meaningless.

Why would anyone bring a warrior?

Again, why not? The fact that, with proper backup, a mail wearer could tank foo doesn't mean a warrior won't be better at it. Put it the other way around, if you have an excess of healers (I know, I know, but for the sake of arguing) you can do with a mail wearer, but bring in a warrior and the healers won't have to keep a neverending stream of heals, possibly freing up time to do something else. You haven't trivialized anything, the way I see it you can still get yourself a party-wipe when Onyxia kicks out your tank (be it mail or plate wearer) and goes after the squishies, if you're not on your toes and someone else is there to stop her. What you can do is give the enemies abilities that are smarter than "Smack tank incredibly hard, party wipe ensues if my smacking is better than their healing". Which goes back to Jarulf's point.

You don't have to eliminate roles, you just have to allow more flexibility filling those roles. Doing a job with someone who is not the "best" at doing it should be harder, but possible. To go back to one of the things you said, it doesn't necessarily follow that encounters should be made with a requirement of n roles that have to always be filled by the best possible character for it, it could have n roles and it should be possible to fill one or several of them with off-rolers at the expense of making the other ones harder, but doable. You don't trivialize anything compared to a perfectly-geared team unless the difference between a primary-X and an off-X is of orders of magnitude (as far as I know, druids and paladins are not as good as priests healing, for example, but a single priest is not as good as 10 paladins or druids doing heals).

And give the enemies abilities that don't depend on having a correct party composition to be overcome, but rather having the players do something smart.

So,

You cannot have a system where anyone can play whatever role they choose and have success without either making the encounters trivially easy for anyone employing the classes in their correct roles, or making all classes equally good at everything, thus obliterating the need for distinct classes or spells or abilities at all.

Yes you can. City of Heroes proved as much. You don't have to let the players win the hardest encounters with a full team of "off-role" players, but you can leave enough wiggle room to have some of these and get away with it.