Is there a uniform measure of "difficulty", that when multiplied by price, will give "profitability of one hour of mining"?
The standard formula to determine the average profit per day for a given MH/s miner on a given blockchain is as follows:

(S/(D*4295))*60*60*24*R*P

Where S is the speed in MH/s rating of the miner, D is the difficulty rating, R is the reward (50 BTC/block for Bitcoin) and P is the price in USD of a single coin.

That I know of there is no standard profitability measure but it would be easy enough to create one given standard values of S. If for example we define the standard as S=100 MH/s then we could easily compare the profitability of mining various blockchains in the resultant USD calculation.