This is the second part in what will be an ongoing series of discussions on feature implementation in MMORPGs, who got them right, and who got them wrong.
Instanced, Match-Based PvP
I believe (but would not bet) that Dark Age of Camelot had the first instanced PvP of an MMORPG in their training battlegrounds. These were discrete zones segregated by level that you could enter and fight the enemy for control of a keep. However, there was no matchmaking and the zone was always sitting there. You could easily enter and find no opponents or allies and thus have little to do. So I'm not counting it for the purposes of this article.
World of Warcraft may have been the first MMORPG to offer match-based PvP instances, but most modern MMORPGs now offer something similar. Just off the top of my head, I know of Warhammer Online, Rift, Age of Conan, Champions Online, Star Trek Online and more.
I believe that Warhammer Online actually had the best match-based PvP of any game currently available. Some of the features they invented (such as queue anywhere) have become industry standards, so I can't be the only one. WAR scenarios managed to be brutal, fast, fun, and surprisingly balanced, but most importantly they managed to make every member of the team have a clear impact, regardless of class. WAR has to be the first MMORPG to make "tanks" feel powerful in PvP rather than just giant punching bags. The use of collision detection was a huge plus, and tactics and strategy were vital to success. You could have fun and be useful no matter your class, and victory was determined by who had the best tanks, healers, and damage dealers, rather than just the best healers (as is more typical).
Scenario design also played a big part in WAR's successful implementation of this feature. With a few exceptions the scenario maps were balanced for both sides, but more importantly the maps couldn't stalemate like say, a Warsong Gulch match back in the day in WoW. The scenarios always kept moving forward, and there was enough variety to make playing scenarios often worthwhile. Unfortunately, the rest of the game wasn't quite up to par, and you can only play so many scenarios before you'd like to do something else. WAR was a flawed game, but the scenario PvP was generally very good.
I think Star Trek Online has done instanced match-based PvP the most poorly, though Champions Online (also by Cryptic) is a strong contender. Although they keep many of the modern conventions such as queing from anywhere, there seems to be little emphasis on balance, and those battles I participated in didn't seem to have any terribly interesting objectives. With no balance and no interesting objectives, teamwork seems to be largely absent as people fly around in seemingly random directions. STO's PvP balance is extremely punishing, with some classes/builds/ships able to dominate while others seem incapable of accomplishing much of anything. Woe betide the newbie who enters an STO PvP match, as they're likely to be brutally ganked over and over with no hope of having fun.
Rift and WoW both have robust matched PvP systems, with Rift's edging out WoW's thanks to adopting and refining many of the things WAR and WoW did. I still have to give top honors to WAR though, for managing to bring it all together in a way no other game did before, or has since.