Before the launch of World of Warcraft (WoW) in 2004, online gaming systems did exist, and there were a number of massively multiplayer online (MMO) systems available. However, with the introduction of a system that is both friendly to new players and has the challenging and addictive qualities that keep long-time gamers interested, Blizzard changed the way that players related to their online worlds, and in doing so, changed the face of online gaming.
Ease of progression
By introducing the quest marker system, WoW ensured that no-one would spend hours simply looking for the next thing to do. Almost all RPGs now use a variant on this system, many directly copying the exclamation mark indicator, which means that players no longer become frustrated repeating conversations with irrelevant NPCs while looking for the next things their characters need to do.
Groupwork through instances
Although a couple of other systems, famously Everquest in one of its expansions, introduced the concept of instances prior to the release of WoW, this was the first game to make it an integral part of the system. This allowed all players to complete quests, encouraging teamwork and cooperation across the globe as teams formed regardless of physical location. The fact that WoW can be essentially soloed also helped with this, allowing players to specifically choose the social interactions they have online rather than being forced into them by the game.
Another factor that increased the sociability of online gaming and reduced barriers to entry was the concept of consensual PvP. Unlike the MMOs that came before it, which were all either no combat or full PvP, WoW allows users to decide for themselves if they wish to attack and be attacked by other players. This protects new players in particular, and has become a standard setting for MMOs.
Reach and coverage
Of course, perhaps the most important way that WoW changed online gaming was simply by making it mainstream and allowing it to gain popular acceptance. It achieved over 11 million subscribers at peak, giving it a breadth and reach no other online game had seen. This normalised the genre; once almost everyone knows someone who plays, online gaming is no longer weird or niche, but rather a standard hobby.