Cryptic Studios has an awesome entry on their blog!  It's called PvP Considerations: Persistence and Instancing.

Some of these trump others. For example, if you wish to give substantial rewards for PvP, you need to establish concrete methods to block the more obvious methods for gaming the system. If you reward directly upon defeating another character, you need to ensure that your system has some method of keeping gank squad behavior in check.

Objective-based PvP, too, is especially vulnerable to class balance. Whereas combat designers will generally spend a healthy amount of time fine tuning class versus class fights, objective-based PvP like capture the flag style games or holding capture points will favor not only certain abilities and classes, but also certain playstyles.

A great look at the technical aspects of game development with regards to PvP.