In retrospect I suppose it's pretty obvious, but it got me thinking about game mechanics and how game designers attempt to use them to influence players. If the information provided by your game mechanics is resisted by your playerbase, that peer pressure will override almost any efforts on your part to encourage certain gameplay.
For example, no matter how much effort you've put into making your classes and builds all useful and balanced, your players may decide that a given build is "wrong." There are lots of stories in WoW and other online games of people having to roll new characters or respec so that they could get into a good guild, or of being snubbed for having a certain kind of "useless" character.
I think the most extreme example I've seen is the prevalence of knife only servers for various games, like Counterstrike and BF2142. The players there are very strict about ignoring all weapons in the game save one, and quickly ban any player who plays the game the normal way.
Peer pressure can be harnessedPeer pressure among players is a problem that I don't think any developers have notably removed, but there are definitely some who have managed to use its momentum in a positive way.
Achaea, a dauntingly complex MUD, has guilds for each of the game's classes, all of which are run by players. Those players get to decide the requirements and initiations for those classes, who gets to select that class, and when. Everyone has a mentor, and tasks to complete, and knows every other person on the server who was part of their class. It's a lot like in-game fraternities or dojos.
By spending any amount of time with one of those groups, players very quickly start to get carried away roleplaying a noble paladin or a devious assassin. Distrust for the enemy groups is also built into that structure, and I remember being completely terrified and excited to start some pvp. It was definitely one of the most interesting social experiences I've seen in a game.
I haven't played it, but I hear that A Tale in the Desert is another game that uses peer pressure to great effect. The players actually pass laws and work together on huge cooperative projects to build pyramids and other large structures. At the end of each "telling" of the game, the players are heavily involved in designing the next version's rules.
It's not always a positive force, and not always a negative force, but player peer pressure in multiplayer games should really be taken into account in the design. It's a sort of wildcard game mechanic that designers have little direct control over once it starts gathering steam, so it needs attention as early as possible.