Duels
At any time in your travels, you may opt to have a 1v1 duel with another player. You do this by selecting and walking up to him, right click on his portrait and choose the Duel option or by using the /duel command. A duel flag will then appear between him and you. If he accepts the duel, the battle will commence; he will turn hostile and his name will change colour to red. During this time, all combat rules specific to your server ruleset will apply, such as the spell limitations of control type spells on the PvP server.
The duel does not end until one of two things occur. When one player's hp is reduced to 1 or less, the result of the duel is announced to the area near the duel flag. No deaths actually occur and the defeated player is simply set to 1 hp. The duelling area is a large area of roughly 50 yards centered on the duel flag. If one player leaves the duelling zone for more than 10 seconds, he is considered to have forfeited the duel. Alternative, a player may also use /forfeit command. The results of this will also be announced to other players nearby.
There is no penalty for winning or losing a duel except that both parties have to spend some time after the battle to heal their hp and mana.
NPC Guard System
Many NPCs in the game will have guards protecting them including quest givers and vendors. Some of these guards are clearly visible while others are summoned only when the NPC is attacked. Some of these guards will keep spawning regularly until the NPC is killed.
Since the introduction of the PvP system some NPCs from the opposite faction will come up with the civilian tag. Killing civilians will now result in a Dishonorable kill.
Guards all have very fast respawn rates. If you want them to stay dead for a longer period of time then don't loot their bodies. As soon as you loot a guards body that body will decay and spawn within minutes. By not looting bodies you can double the re-spawn time.
Paper, Scissors, Rock
Game balance is of the utmost importance to Blizzard and they're approaching PvP with a rock, scissors, paper type solution so some classes will fare better against others in combat. It sounds unfair but Blizzard have made this decision to encourage teamplay and guild formation. A mixed team will always fare better than a team with only one character class. An example of this is that Priests can beat a Mage fairly easily however Priests find it much harder versus a Shamen.
This also allows for exciting gameplay as unlike other fast paced 3D multi player killing games it means that everyone doesn't have the same armor / weapons. It offers an entire new platform of tools to players to offer much more depth to player versus player combat.