Dark Age of Camelot (DAoC), released in 2001 by Mythic Entertainment, was really the first MMO to be released that focused on PvP as the primary endgame activity. The King Arthur mythology-themed game allowed players to choose from three different factions: Albion, Midgard and Hibernia, with dozens of races and classes at their disposal. Players could only group with members of their faction, and the world was split up into various pieces to allow the different factions to level up without getting into each other's hair too much.
Once players reached level 50, they would compete for magical relics at various keeps in battlegrounds. The numbers were not kept balanced by the game, and the fact that the realm populations were rarely kept at parity made it so that there were perpetual balance issues. In addition, the fact that the PvE portion of the game took so long and was so stultifying turned many players off from the game that might otherwise have been interested. Later on in the game's development, the developers made leveling easier and made it possible for players to grow their characters by participating in lower level battlegrounds; it highlighted a critical issue that many other developers have learned from since.
PvPers don't like being pushed into doing things that they don't enjoy so that they can perform better in PvP. On the other hand, many players like the concept of having to "pay your dues" before you're allowed to play with the big boys. From the developer's standpoint, it's something of a mixed bag. They want the players to invest more time into their product so that they feel more resistance to canceling the game; but they also don't want them to feel so bored that they don't subscribe beyond their first free month. They like that their higher level players have a feeling of accomplishment and superiority, but not so much that they put down newbies that aren't as accomplished as they are.
The release of the Trials of Atlantis expansion in 2003 was a major turning point for the game and ended up alienating many of their subscribers. It introduced a great deal of raid content in the hopes of attracting more PvE players from other games and retaining those that they had. The fact that PvPers now had to invest a great deal of time into raiding in order to compete effectively ended up alienating many of the players and eroding DAoC's competitive advantage, which was always high intensity mass PvP combat.