Although modern MMORPGs sometimes differ dramatically from their predecessors, many of them share the same basic characteristics. These include several common features: persistent game environment, some form of level progression, social interaction within the game, in-game culture, system architecture, membership in a group, and character customization.
The majority of popular MMORPGs are based on traditional fantasy themes, often occurring in an in-game universe comparable to that of Dungeons & Dragons. Some employ hybrid themes that either merge or replace fantasy elements with those of science fiction, sword and sorcery, or crime fiction. Still, others draw thematic material from American comic books, the occult, and other genres. Often, these elements are developed using similar tasks and scenarios involving quests, monsters, and loot.
In nearly all MMORPGs, the development of the player's character is the primary goal. Nearly all MMORPGs 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 non-player characters, either alone or in groups, are the primary ways to earn experience points. The accumulation of wealth (including combat-useful items) is also a way to progress in many MMORPGs. 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. Eve Online trains skills in real time rather than using experience points as a measure of progression.
In some MMORPGs, there is no limit to a player's level, allowing the grinding experience to continue indefinitely. MMORPGs that use this model often glorify top ranked players by displaying their avatars on the game's website or posting their stats on a high score screen. Another common practice is to enforce a maximum reachable level for all players, often referred to as a level cap. Once reached, the definition of a player's progression changes. Instead of being awarded primarily with experience for completing quests and dungeons, the player's motivation to continue playing will be replaced with collecting money and equipment.
Often, the widened range of equipment available at the maximum level will have increased aesthetic value to distinguish high ranking players in game between lower ranked players. Colloquially known as endgame gear, this set of empowered weapons and armor adds a competitive edge to both scripted boss encounters as well as player vs player combat. Player motivation to outperform others is fueled by acquiring such items and is a significant determining factor in their success or failure in combat-related situations.
MMORPGs almost always have tools to facilitate communication between players. Many MMORPGs offer support for in-game or clans, though these will usually form whether the game supports them or not.
In addition, most MMOGs require some degree of teamwork in parts of the game. These tasks usually require players to take on roles in the group, such as protecting other players from damage (called tanking), "healing" damage done to other players or damaging enemies.
MMORPGs generally have Game Moderators or Game Masters (frequently referred to as GMs or "mods"), who 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.
Relationships formed in MMORPGs can often be just as intense as relationships formed between friends or partners met outside the game, and often involve elements of collaboration and trust between players.
Most MMORPGs provide different types of classes that players can choose. Among those classes, a small portion of players choose to roleplay their characters, and there are rules that provide functionality and content to those who do. Community resources such as forums and guides exist in support of this play style.
For example, if a player wants to play a priest role in his MMORPG world, he might buy a cope from a shop and learn priestly skills, proceeding to speak, act, and interact with others as their character would. This may or may not include pursuing other goals such as wealth or experience. Guilds or similar groups with a focus on roleplaying may develop extended in-depth narratives using the setting and resources similar to those in the game world.
Over time, the MMORPG community has developed a sub-culture with its own slang and metaphor, as well as an unwritten list of social rules and taboos. Players will often complain about 'grind' (a slang term for any repetitive, time-consuming activity in an MMORPG), or talk about 'buffs' and 'nerfs' (respectively an upgrade or downgrade of a particular game mechanic).
As with all such cultures, social rules exist for such things as invitations to join an adventuring party, the proper division of treasure, and how a player is expected to behave while grouped with other players.
Debate rages in various gaming media over the long-term effect of video game overuse. The On-Line Gamers Anonymous forums are filled with stories of players that have neglected social, employment and/or family responsibilities in favor of their virtual lives.
Most MMORPGs are deployed using a client–server system architecture. The server software generates a persistent instance of the virtual world that runs continuously, and players connect to it via a 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 Guild Wars 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.
Some MMORPGs require payment or a monthly subscription to play. By definition, "massively multiplayer" games are always online, and most require some sort of continuous revenue (such as monthly subscriptions and advertisements) for maintenance and development purposes. Some games, such as Guild Wars have disposed of the 'monthly fee' model entirely, and recover costs directly through sales of the software and associated expansion packs. Still, others adopt a micropayment model where the core content is free, but players are given the option to purchase additional content, such as equipment, aesthetic items, or pets. Games that make use of this model often have originated in Korea, such as Flyff and MapleStory. This business model is alternately called "pay for perks" or "freemium", and games using it often describe themselves with the term "free-to-play".
Depending on the number of players and the system architecture, an 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; World of Warcraft is a prominent example, with each separate server housing several thousand players. In many MMORPGs the number of players in one world is often limited to around a few thousand, but a notable example of the opposite is EVE Online, which accommodates several hundred thousand players on the same server, with over 60,000 playing simultaneously (June 2010) at certain times. Some games allow characters to appear on any world, but not simultaneously (such as Seal Online: Evolution), others limit each character to the world in which it was created. World of Warcraft has experimented with "cross-realm" (i.e. cross-server) interaction in player vs player "battlegrounds", using server clusters or "battlegroups" to co-ordinate players looking to participate in structured player vs player content such as the Warsong Gulch or Alterac Valley battlegrounds. Additionally, patch 3.3, released on December 8, 2009, introduced a cross-realm "looking for group" system to help players form groups for instanced content (though not for open-world questing) from a larger pool of characters than their home server can necessarily provide.