Aha! Recently studied this.

The reason for so much of Hannibal's success was based on his knowledge of the Roman consul's temperaments. The consuls alternated command each day, so Hannibal could essentially choose who he wanted to fight. Varros, one of the Roman consuls, was very aggressive and was basically itching to take a shot at Hannibal. Hannibal knew this and used it to plan his attack. I get a strong sense that the majority of it went exactly according to plan for Hannibal, as the ensuing slaughter was just ridiculous. Of course, battle is chaos so I'm sure that tweaks were made on the fly, but I'm not confident enough in my knowledge to make any claims. The Romans had a notoriously weak cavalry throughout history and were very infantry-based, leading to the quick annihilation of the Roman flanks. The concave front line that Hannibal used to suck in the Roman infantry helped eliminate any support for the Roman cavalry. Once the Roman cavalry dispersed, surrounding the remaining Roman infantry was fairly simple. What I find interesting is that initially Hannibal's infantry was in a convex formation, and they essentially pulled back with the Romans while the flanks held, leading to the Romans envelopment.

This website gives an excellent illustration: http://www.roman-empire.net/army/cannae.html