A persistent world is a specific kind of system. All massively multiplayer RPGs are persistent worlds, but not all persistent worlds are MMORPGs. For now, let's stick to MMORPGs.
A persistent world basically says that whatever a player accomplishes in one session carries on to other sessions. So, if you gain a level, you'll still have that level next time you log in.
Players who play a lot will almost always have more, better stuff than players who play less, simply because every time you play you get a little bit more. Once you have a little more, you never lose it, because the world is persistent. If everyone gains a level every hour, someone who plays eight hours a day will have eight times the level of someone who only plays one hour a day.
The problem with this appears when players overlap. If anyone can attack anyone, the guy who plays eight hours a day will probably kill the guy who only plays one hour a day. This is commonly called "being a dick", although in the games industry we call them "griefers". This gets especially bad when there's a reward for killing players - like XP or items.
Most MMORPGs, therefore, strictly limit how players can interact. People can only fight people within three levels of them, or while they're in the arena, or something. High-level quests don't overlap low-level quests, so the low-level guys don't have their thunder stolen. You can't equip over-level items, so that you can't be unnaturally powerful for your carefully defined level bracket...
But these were not the first step. These were the last step. If preventing players from screwing each other was the same as building a house, that last paragraph would be what color you paint the walls.
Back in the day, there were quite a few text-based on-line games that let you create stuff. Even now, you can find them. Originally, there were few limits on what you could create. Create a space ship, a hundred room dungeon, a monster that can eat planets. Create anything you like, if you can figure out how to script it into the game world.
People quickly realized that this was not a terribly good idea for any number of players over, say, three. One player would program a death star and blow up another player's fairy castle. Much
So they started to figure out ways to limit the players. They tried "wizards" - users who had the power to police content. They tried a points system, so only people with experience and favorable ratings could create content. They tried limiting the capabilities of the scripting engine, PvP restrictions, limiting your content to "your land", keeping it out of public spaces... many other things.
The problem was, the more players play the game, the more brains are thrown against the scripting language. Every limit reduced the chances of a player accomplishing something dastardly, but more players meant more chances to roll snake-eyes and get a bastard who figured out how to make everyone else miserable.
Games like World of Warcraft have MILLIONS of players. So the limits have to make it MILLIONS of times harder for players to make other people miserable.
This means that the limits have to be really, really strict. Basically to the point of cutting the scripting language out entirely. However, at that level, the background noise is very loud. Meaning that players writing scripts is not the problem: there's a lot of other ways to grief people, ways that would not really occur to someone with the ability to build a death star to solve his problems.
Things like ninja looting, path obstructing, training, item duping, selling accounts, gold farming, min-maxing, and naming your characters "iluvtits" and "wolverine".
Compared to having your planet blown up, these are relatively minor things. But they're still very irritating.
Now, some games have decided to weather the shitstorm and leave a kind of scripting language in their game, with the understanding that the world will have more than its fair share of grief.
One example is SecondLife, which actually
Obviously, Eve Online has a more limited system so, in general, it's harder to cause grief. But it's much, much easier than something like WoW, and sometimes it's truly spectacular, like one hundred-player faction betraying and completely annihilating another hundred-player faction, stealing (real-world legal) tens of thousands of US$ worth of in-game assets...
SecondLife started off with an extremely open language, and that meant that players found it easy to grief each other. When it was young and small, this was not such a terribly big deal: if someone made a fuss, the GMs cleaned it up.
But the player base has expanded and expanded and expanded. Where there was one griefer, there are now a literal thousand.
They couldn't really handle this, so they've been following the history of old-school multi-user dungeons: implementing limits. Ratings limits, land restrictions, crippling the scripting language, restricting the number of visual elements, requiring a credit card...
I don't know if there is a solution, but I have a hunch it involves dramatically lowering the population density of a game.
Anyhow, there's the bell. I hope you've enjoyed the session. If you notice any errors in the text book, speak up. Read chapter two for tomorrow, and do the questions on page 37 for homework. There will be a quiz.