A better model might be the Savage Worlds damage threshold system (in M&M terms, it is a variant in the Mastermind's Manual called "Damage Roll" or something like that).
Basically, the attacker rolls damage normally. Instead of subtracting hit points, though, you compare the result to a threshold. If you're under the threshold, nothing happens (or, something very minor, like -1 to the threshold until the target can take a short rest). If you're over the threshold, something bad happens to the target, like they take a wound. For each X points over the threshold, they take an additional wound, and when they exceed Y wounds, they drop.
In Savage Worlds, X is 4 and Y is 3 (for important characters, and 0 for minions). There's also a level before wounded (basically you become dazed (save ends) unless you are already in that state, in which case you take a wound). But those are details of the Savage Worlds implementation. The core concept -- compare the damage to a threshold instead of subtracting it from HP -- is what's important.
I think the difficulty in such a rule is coming up with a reasonable number for the threshold. You want it low enough so that everyone has a decent shot at exceeding it with an at-will attack. But you want it high enough so that nobody can hit it automatically with an at-will attack. And the margin required for additional wounds should also be high enough to account for crits and daily powers and so forth.
-- 77IM