Some very valid points mentioned in the post above by Procricket, Jon and PM7 regarding primarily the rebound performance of the bat. I like to think of the overall performance being a combination of 3 factors:
1) How good the middle is? - this point has been debated well above and I think Jon's 5 points are spot on
2) How frequently you find the middle? - this has a lot to do with the batsman's awareness of his own game and consequently what deadweight and "pendulum" swing is best suited for you.
3) How forgiving the off-middle areas are? In other words the consistency and uniformity of the rebound over a larger area whether that rebound was great or average.
Looking back at my own bat buying experiments, I would say that the key difference between the best bats I had and the worst was on 2) and 3). And grade, grains, price etc. had little to do with it. THe best 4 bats I have ever had are a medium range SS, MB Sher, Screaming Cat and M&H Distinction. That'll give you a pretty awful correlation chart !