Not quite as elegant as e=mc^2, but then again, that equation appears to be wrong near the speed of light anyway.
Submitted for discussion, the law of mob pull difficulty.
Where D = Difficulty Factor
A = Situations DPS need to be aware of to avoid death
B = Required Actions By DPS
C = Buttons that DPS can hit that result in any undesired result
H = Hunters in the group
K = Deathknights in the group
D = (A + B + C + 2H + 3K)/100
So if a given pull requires 1 crowd control, 3 spots on the ground that can kill you if you stand in it, and 60 spells that can break said necessary crowd control, with 1 hunter and 1 deathknight, we have a Difficult Factor of 1+3+60+2+3 or .69
On the other hand, a pull with no dk's or hunters, no required crowd control, nothing on the ground, and no spell reflection or other bad thing that can happen when dps simply unload mindlessly, we have a 0 difficulty, meaning a tank and healer can handle it alone and the dps are only along for the ride to speed things up.
For discussion and enhancement purposes to the difficulty factor equation, possible additional variables might include modifiers for propensity of healer to allow dps to die (which, if high, would reduce the difficulty factor), a factor for paladin tank compassion - meaning how likely is he to bubble a dps to prevent him from killing himself/the group, and perhaps a summation equation to include the difficulty of the entire dungeon.