I kinda like Vroomfongle's thought process...
A melee attack is supposed to be swinging, jabbing and striking over the 10 second round. You roll to see if your successful with one of these, however if you roll over 150, you may be successful at landing two blows during those 10 seconds (without the assist of haste)
The strength of any given blow is described by the weapon chart, but the severity is described by the crit. Thus a double open-ended 250 attack roll could me 2 or even 3 hits on the target, and while at least 2 of them landed pretty soundly, they aren't necessarily all severe blows... that's for the crits to decide.
I am however troubled, because I think that a higher level warrior should find it easier to land killing blows then a lower level warrior (more-so then is reflected in the difference between an A and an E crit)
Here's a thought:.. An all out murderous attack. This haymaker of blows could have to land in the deliberate phase, with no parrying allowed, further it would be somewhat obvious, giving the opponent a situational awareness roll, to see it coming and decide to abort actions and escape the blow, since the PC's not jabbing, and slashing, but rather winding up the opponent might not be a need to 'break from melee', there may have to be some sort of a speed contest though.. Finally, when swinging, the PC would declare how much of his OB he's sacrificing for raw power. Assuming a crit lands, every 10 OB sacrificed moves the crit down a box on the table.
This kind of steps on the ambush skill (since anyone could do this when surprising someone), but I always thought ambush was overpriced and mostly unnecessary. If you sneak up on someone and are competent with your weapon, you should be able to effectively put it in the back of their skull without any additional skills.
Thoughts?