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Watch the whole thing, but specifically from 6:45 mark.
https://www.youtube.com/watch?v=Ut4riV8MjJw
Obviously up to interpretation, but my view is that you can't be "the hero" facerolling the world as some are assuming due to the kitty spinning in circles.
If you or some friends come across a dragon, you can't just run up, mash buttons, and win.
Scaling: Unless this has been clarified, I don't take it as a dragon's attacks/health change the more people show up.
I take it as mobs still have "levels" by their difficulty. A kobold may be lvl 1 and a dragon lvl 200. Anyone is free to try and kill them both, but most likely the dragon is going to have you for lunch.
A kobold alone is lvl 1 and easy, but 20 of them is equal to lvl 20 and isn't as easy. You can't just run in and faceroll 20 as easy as 1 (demo from reveal what just that, a demo).
So while there isn't a traditional level scale, there is a difficulty scale and depending on how good you and your friends are, it may take 10 people or 100. Not that the game changes to make it easier/harder depending on who shows up to the fight.
Trinity/Teamwork: Particular builds will still be needed in some way. You can't just brute force through the entire game.
I didn't do any raiding past POP in EQ, but beforehand and my time in WoW, I was in many battles that required skills from all classes.
If 20 adds come in while fighting a dragon, you need CC to control them. If a deadly DOT starts ticking and isn't removed quickly, everyone dies, a healer is needed, etc. Maybe a mob is immune to particular weapons or spells, adjust accordingly.
If we showed up to Naxx with 40 warriors, we would of be facerolled ourselves. We were wiped so many times with the "perfect trinity" and then some.
For a FPS version, I play a lot of Team Fortress 2 that allows you to change your "class" quickly. You can have 16 heavies shooting the heck out of everything, but add in a medic, sniper, pyro, etc and the team dynamic and your chances of winning increases very quickly. While I've seen a team of all snipers completely destroy another team, I've see a team with 4 snipers not get out of spawn. Even in a "faceroll" type game, teamwork is still the winning element.
Raiding: All of this gives us "raiding". Not the exact traditional, meet at this location, at this time, with this build, wearing this gear, but very similar just with a lot more individual freedom.
Again this is all my opinion, but until we get a bit more information and can see some of this in action, the sky isn't falling yet.
People that want a challenge should still find one. The difference is you will actually have to "find" it and then use your brain to figure out how to win. You can't just show up with X tanks X healers X dps and assume you can win. The tried and true trinity has its place still, just with a lot more options.