Interesting idea. I'm not sure if I like it, and it may be too complicated (especially when you have a "creature" that still has hp but no bodies that can use that hp), but it's certainly an interesting idea.
I'll echo this. I don't know if I like it, but its one of the more innovative ideas I've seen on this subject.
From listening to the discussions so far, I would say that the group thought seems to worry most about autodamage. I think larger area effects at higher levels are fine. I mean, if my fighter takes all solo killing powers, fighting solos for the group is going to be easier. If the wizard takes big area effects like fireball instead of more direct damage powers, he's going to be better at killing minions.
With that in mind, I like both the save to resist autodamage or the bloodied when first damaged, dead on second ideas. I personally do not like the damage threshold idea, mainly becomes its biased against certain kinds of autodamage. A rain of blows might still purge through minions but the pact blade doesn't, etc.
So far I've been firmly on the save to resist camp, but I'm warming up to the bloodied idea. While I don't like tracking, I think in some ways its faster for me to bloody all the minions that just took autodamage without having to worry about a lot of rolls.
And finally, I think victim is spot on that perhaps the problem with minions isn't the basic minion concept, but their current design in the MM. Ranged minions that scatter don't have the problem facing rain of blows, flaming sphere or most autodamage effects near as much as as melee ones. Minions that have effects when they die are still important to the combat even if they are easily dispatched, etc. I think the same is true with solos, there are solos in the MM I think are flat out better than others.
While I'm always interested in system tweaks to improve play, this may be a case of using the current system rules and simply creating better minions.