Everyone knows World of Warcraft. Some have played it and, of course, have become addicted. But few people realize the real reason World of Warcraft is so popular. It can't be the game itself, because it's just another grind-fest for levels and loots. So what makes World of Warcraft so popular?
First, it's the convenience. Everything is there to make the game more enjoyable. For fast travel, players have a choice between the free goblin zeppelins, the pay-per-route air travel, or walking (with or without mount). There's a few other things, of course, like some classes getting 'travel forms', which allow them to travel faster, with the cost of some (most) of their other abilities. And there's the heartstones, to teleport back to a pre-selected inn. And the warlock portal. In short, time is almost never wasted in World of Warcraft. Players can play almost non-stop, which is a reason why it's so popular.
Second is the thoroughness. Everything has been thought of. There's mailboxes for sending messages and items to other players, auction houses to prevent sell spamming, meeting stones for forming parties for instanced dungeons (although those have met moderate success), quest markers to find and complete quests faster and even profession skills that help you find harvestable resources (herbs and ores) on the minimap.
Finally, you have the addiction factor. While it is relatively easy to reach level 60 (maximum level until the expansion) (compared to other MMORPGs), there's always something more to do. A lot of players will feel compelled to spend day after day trying to get every piece of equipment they can get their hands on. Others will go to battlegrounds to gain honor, or go grief lower-level players (in player-versus-player servers). Those who aren't compelled to any of those can start new characters; Blizzard has made sure that everything you do, even if you've done it a dozen times before, holds at least some interest.
But when you look at the game itself, there's very little in the way of innovation. The game is divided in races and classes - 8 races, 4 for each side of the conflict, and 9 classes, with each race having the choice of only a few of them. Players choose within a range of talents to further differentiate their character from others of the same class, but there aren't that many sets to choose from. In addition, each player can choose 2 professions (in addition to the 3 secondary ones, which are essentially free), though usually, one will choose a profession which suits their class well.
When you look at all this, you realize that the success of World of Warcraft can be recreated; furthermore, it can be recreated without looking like plagiarism, since most of the things WoW did well aren't noticeably part of the game's mechanics. So when you create your own MMORPG, don't try to emulate World of Warcraft too much. The main game mechanics are probably more limiting to players than they should be; it's the smaller, less noticed aspects of the game that gives World of Warcraft its sharp edge that has yet to be emulated fully.