First and foremost is Communication. This may seem obvious, but the devil lay in the details. Communication is more than a relay of information. Many things impact communication:

Choice of words
Volume
Proximity
Sarcasm
Language, culture, nationality, etc
Medium (letter, email, in person, etc)
External noises
Social media (facebook, twitter, reddit, etc)
Body language
Relationship with other party
and Listening (more specifically, Active Listening)

Active Listening signals that the listener is actually listening. Rephrasing is a method of active listening and is much more effective than saying, "I'm listening". Most, if not all, conflicts can be attributed to some degree of communication issues. The basic problem is that we automatically assume we understand what the other is saying, why they are saying it, and what it all means. With the speed of communication these days we spend less time listening and, therefore, do not fully understand the information relayed…but we think we do.Communication has its own importance, but it also extends to the next influence on conflicts: Trust