I have wondered about this in the past. We never see Darth Vader use Force Lightning and he was easily put down by Palpatine with Force Lightning in ROTJ. Yet in EP2 Count Dooku uses Force Lightning. In the OT Vader was more powerful than Dooku ever was. Yet, no Force Lightning. Not only that but he was pretty messed up by the Emperor.
I had speculated that it was because of his "machine" parts.
Turns out that is exactly right. According to the new "Visual Dictionary" because of his mechanical arms Darth Vader can never use Force Lightning this also means that we will always be vulnerable to Force Lightning.
So there you have it.
This explains a great deal of how Palpatine manages to keep Vader under his thumb for all those years.