Serial Ganker sid67 wrote an interesting blog post on the many faces of PvP. I do agree that there is not *one* PvP, but that for example battlegrounds, arenas, and castle sieges are very different from each other, not only in gameplay, but also in purpose and to what kind of player they appeal.
I don't agree that "griefing" is not an important motivational factor in PvP. While sid67 is right that you can grief players of your own faction by blocking the mailbox or similar stunts, PvP griefing does happen all the time. Even me, playing on a PvE server, just this weekend got griefed by some enemy faction rogue who found it funny to kill all the auctioneers in the city where my bank alt was parked. My limited attempts on PvP servers ended with me being corpse camped by somebody many levels higher than me, until I just gave up.
So why do some people like the ability to grief so much? The answer, curiously enough, lies in the original Everquest, a game which did not have PvP on the overwhelming majority of its servers. But what EQ had was extremely harsh PvE, with extreme "impact", as sid67 calls it, when you lost. As a reaction, people banded together *against the game*, to survive. That is a basic human instinct, going back to the Neanderthals: When things get rough, you better find some friends to help you. The negative consequence of "no impact" PvE in games like World of Warcraft was that it degenerated MMORPGs into massively single-player games. Thus the fans of impact PvP, who hope that introducing impact PvP can replace impact PvE in its function to make people band together.
Unfortunately that simply doesn't work. Everquest worked because at the time there weren't so many alternatives, and the existing alternatives weren't any less harsh. But the so-called "Vision" of bringing back a harsh environment, PvE or PvP, to force people to band together against it, nowadays only results in that harsh game becoming extremely niche, with less than 100k subscribers. The large majority of MMORPG players simply do not accept harsh any more.
PvP in MMORPGs is further hindered, as sid67 alludes to, by the fundamental problem that fairness in PvP is incompatible with the basic game principle of character development in MMORPGs. Take away characters getting stronger through gear and levels, and you don't have a MMORPG any more. Allow character development, and you'll always end up with a huge range of problems of stronger characters "ganking" less strong characters. And, again sid67, the more "impact", that is negative consequences from losing, PvP has, the more likely it is that the loser will find himself in a downward spiral. In the real world a cornered rat might turn into a ferocious guerilla fighter. In a virtual world the rat just logs off and either creates a new character on the winning side, or quits the game. Thus you end up with the somewhat perverse notion of "reverse impact PvP", where the *winning* faction gets penalized in the next round of fights.
The only viable solution for PvP in mass market MMORPGs is positive sum PvP, with basically no impact at all, but with both factions being rewarded for participating. In that option the rewards are totally artificial, handed out by the devs so to say, and thus the developers can reward the kind of PvP which is least likely to drive other customers away. In games which have both that sort of positive sum PvP *and* impact PvP, the majority of players will drift towards the positive sum version, which is one of the reasons why WAR failed. Ultimately game developers need to consider the question of whether the old mantra that "players want PvP" is really true, if you can only get them to participate in PvP by handing out better rewards than for PvE.
[EDIT: As if to prove my point, SOE just announced battlegrounds for EQ2.]