I don't mind the autocrit on a 20 in principal, again we know next to nothing about the system. Characters might be able to spend manuevers or even an action point or something to negate crits, who knows!!
As for the group of mooks only hitting on 20s and therefore always critting, I don't have a problem with that. As others have mentioned the damage is still not terribly high, and it keeps mooks a challenge longer. That is nice for dms because making mooks is easy, and playing them is easy. I just roll 10 d20's, any 20's? Nope, alright you guys do your awesome thing.
Having said that, it would take a lot more work to do that for DND than for SAGA. Dnd has a nice varied amount of crit ranges to work with. But on the other hand, we already have reports that weapons may have new abilities to differentiate themselves. If that's true, the need for crit range variation goes away.
As to the idea of casters roll 1 attack for a group of creatures, I don't think that will work in dnd. It works great in SAGA, but SAGA is a low magic game. As cool as the force may be, it doesn't hold a candle to the stuff magic can do in dnd. Besides, I think players like to roll die.
Example, Dm throws a dominate at a player. If the player rolls the die and fails, his die is cursed, his luck has ended, and he takes the dominate. If the dm succeds, the dm is an ass, he's targetting the player, etc. Not all players feel that way, but they like to feel that they have control over the destiny of their character, even if the math is exactly the same, and there feeling is a complete illusion....well that's what fantasy is for right?