Common features

Though MMORPGs have evolved considerably, and modern versions sometimes differ dramatically from their antecedents, many of them share some basic characteristics.

Themes

The majority of MMORPGs are based on traditional fantasy-themed game play, occurring in an in-game universe comparable to that of Dungeons & Dragons. Some employ hybrid themes that either merge or substitute fantasy elements with those of science fiction, sword and sorcery, or crime fiction. Still others use more obscure themes, including American comic books, the occult, and other recognizable literary genres. Often these elements are developed using similar tasks and scenarios involving quests, monsters, and loot.

Progression

A highly-progressed character from EverQuest 2, with mount.In nearly all MMORPG's the development of the player's character is a primary goal. Many titles feature a character progression system in which players earn experience points for their actions and use those points to reach character "levels", which makes them better at whatever they do. Traditionally, combat with monsters and completing quests for NPC's, either alone or in groups, is the primary way to earn experience points. The accumulation of wealth (including combat-useful items) is also a way to progress in many titles, and again, this is traditionally best accomplished via combat. The cycle produced by these conditions, combat leading to new items allowing for more combat with no change in gameplay, is sometimes pejoratively referred to as the level treadmill, or 'grinding'. The role-playing game Progress Quest was created as a parody of this trend..
Also traditional in the genre is the eventual demand on players to team up with others in order to progress at the optimal rate. This tends to force players to change their real-world schedules in order to "keep up" within the game-world. Though some titles recognize this trend as a problem and provide ways to progress within short, unscheduled periods of time, this is still widely criticized of games in the genre.

Social interaction

MMORPGs always allow players to communicate with one another. Depending on the other interactions allowed by the game, other social expectations will be present.
Many MMORPGs exploit their players' social skills and offer support for in-game guilds or clans (though these will usually form whether the game supports them or not). As a result many players will find themselves as either a member or a leader of such a group after playing a MMORPG for some time. These organizations will likely have further expectations for their members (such as intra-guild assistance).
A Sand Giant and a group of players engaged in combat in 1999-era EverQuest'Even if players never join a formal group, they are still usually expected to be a part of a small team during game play, and will probably be expected to carry out a specialized role. In combat-based MMORPGs, usual roles include the "tank", a character who absorbs enemy blows and protects other members of the team, the "healer", a character responsible for keeping up the health of the party, and the "Damage Dealer," a character specializing in inflicting damage. Other common roles include being a dedicated "buffer" or "debuffer", using abilities that affect the team or the opponents in other ways. Any given MMORPG might allow players to take on all of these roles, additional hybrid roles, or none of them. Despite the variability, some players might enjoy one role over others and continue to play it through many different MMORPG titles.
Some MMORPGs also may expect players to roleplay their characters - that is, to speak and act in the way their character would act, even if it means shying away from other goals such as wealth or experience. Most MMORPG players never actually play the roles of their characters, however, and so this behavior is far from being the norm. Still, MMORPGs may offer "RP-only" servers for those who wish to immerse themselves in the game in this way.
MMORPG's generally have Game Moderators or Game Masters (frequently abbreviated to GM), which may be paid employees or unpaid volunteers who attempt to supervise the world. Some GMs may have additional access to features and information related to the game that are not available to other players and roles.

Culture

Since MMORPGs have so many elements in common, and those elements are experienced by so many people, a common culture of MMORPGs has developed which exists in addition to the culture present within any given game. This usually manifests itself in common terminology. For example, since MMORPGs often feature many different character "classes", the games must be balanced in order to be fair to all players, and this has led players of many games to expect "nerfing" or "buffing", which are terms describing the weakening or strengthening of a subset of players, respectively.
As another example, in many older MMORPGs the fastest way to progress was simply by killing the same monsters over and over again, and as this is still common in the genre all MMORPG players know the process as "grinding". The importance of grinding in MMORPGs, and how much "fun" it contributes to the experience, is constantly debated.
MMORPG addiction, which has been a source of concern for parents, also affects the culture. Some players might look down on those who invest huge amounts of time into a game, while others might scorn those who can't put in the time to "play properly." In 2005, China announced a controversial system of limiting gameplay to three hours per day, and players who played past that would find their character weakened.
Some players have jokingly viewed real life as an MMORPG.

System architecture

Most MMORPGs are deployed using a client-server system architecture. The software that generates and persists the "world" runs continuously on a server, and players connect to it via client software. The client software may provide access to the entire playing world, or further 'expansions' may be required to be purchased to allow access to certain areas of the game, Everquest and World of Warcraft are two examples of games that use such a format. Players generally must purchase the client software for a one-time fee, although an increasing trend is for MMORPGs to work using pre-existing "thin" clients, such as a web browser. A notable example is Runescape, which players connect to via a browser, allowing access to the game independent of platform and location.
Some MMORPGs require payment of a monthly subscription to play. By nature, "massively multiplayer" games are always online, and most require some sort of continuous revenue (such as monthly subscriptions and advertisements) for maintenance and development. Games that feature massively-multiplayer functionality, but do not include roleplaying elements, are referred to as MMOGs.
Depending on the number of players and the system architecture, a MMORPG might actually be run on multiple separate servers, each representing an independent world, where players from one server cannot interact with those from another. In many MMORPGs the number of players in one world is often limited to around a few thousands, but a notable example of the opposite is EVE Online which accommodated around 200,000 players in the same world as of August 2007.