Listening.

I find questioning the importance of listening to be rather ridiculous, but what's more ridiculous is how much said importance is ignored. It is one of the only two critical factors of communication, when you break it down. If you aren't listening to what's communicated, it was all in vain, no matter how affective the communicator was. Likewise, the one doing the communicating is doing everything they can to get you to listen. It's the most important part of the entire process.