Speed, power and stamina. Cotto can't handle power, imagine power with insane speed and constantly there all the way to the 12th. This is a nightmare for Cotto more than Pac.
I have a feeling negotiations will be tough given that Arum and Mayweather hate each others guts. The better Pac-Cotto does, the higher the purse split demands for Pac/Cotto (whoever wins) and the higher the purse split demands, the more difficult the negotiations IMO.