It wasn't fully implemented in Beta, but it has a lot of people hoping for it to show up in release.
Its a system of dictating if you should or shouldnt kill someone. If someone is too low of a level to be of a challenge to you, than you will gain dishonor for "ganking" him. If the person is around your level or higher, you will gain honor based on his level.
When it is fully implemented it will have rewards that can be bought with honor points, and even titles and tabards which show how honorable you are.
Also, if you gain too much dishonor, you will face higher prices at stores and eventually will be KoS by your own faction NPCs.