In my opinion killers (PvP) and explorers (PvE) are the prestigious players who make MMOs great
Explorers aren't really neatly categorizable as PvE like that. The group that is actually the core PvE base are the achievers you malign. They're the ones who want to conquer all the PvE stuff, get the best loot, etc. Explorers are your theorycrafters and dataminers, who may or may not put what they dig up into practice, but won't ever do so with the fervor that Killers and Achievers will. Achievers love trophies: They'll grind a thing over and over, each time trying to best their previous record. An Explorer won't do this: He'll come up with a new paradigm, put it to the test and blow the previous record out of the water, and then having seen what he wanted to see out of it, go find something else to do while the Achievers will then copy the method and push the envelope on that particular paradigm, until the Explorers come up with a new one.
Killers hate Explorers. For the core Killer, fighting an Explorer is something they see as unsatisfying because Explorers, in their eyes, cheat. Explorers rarely bother to enter the PvP arenas unless they have some theory to test, and so killing one doesn't really provide a satisfying experience to a Killer because the Explorers don't fight back sufficiently, nor get particularly upset, while they have this unpleasant tendency to pull out some non-standard surprise that suddenly kills them, something that Killers may see as cheap, unskilled, or unfair. Explorers in PvPs tend to fight using mechanical trickery and rules exploits, and Killers HATE that. A Killer-Explorer tends to be known as a Cheesemaster, the kind of people who discover and perfect the usage of the fromage in all of its cheesy goodness.
socializers are just "there"
Read: The CASUALS and the ERPists.
If you want a good MMO you should design it in a way that will alienate achievers so they can't ruin the damn game for everyone else. Horizontal progression and skill-based combat are good ways of doing this. Often if an achiever reads the words "low level cap" he will fuck right off and never even buy the game, and the community is better off for it. Open world PvP is another great means of dealing with achievers as it means that killers (peace be upon them) will be able to interrupt their grinding by murdering them repeatedly until they leave.
Yeah, that's not how it works. In a sustainable game, you sort of need a community of all of them. Allowing any one group to become too dominant will set off a death spiral that dooms the game.
Open world PvP is another great means of dealing with achievers as it means that killers (peace be upon them) will be able to interrupt their grinding by murdering them repeatedly until they leave.
No, no, that's a failure state in the game. Achievers are the natural prey of the Killers in an Open PvP environment. If all of the prey gets wiped out, the killers will soon become bored and leave. Killers like to murder Achievers, because Achievers are typically well-equipped and put up a decent fight, but ultimately not tuned with the mindset to win, and dislike being killed enough to grant a level of satisfaction to killing them. Achievers in such an environment regard Killers as a sort of challenge, that adds some level of weight to the achievement process, but too many will ultimately drive them away and cause a death spiral. There's a sort of natural predator-prey relationship between Killers and Achievers.
