WoW is not alone. MMPORGs, MUDS and a few multiplayer games (Diablo II comes to mind) are generally very addictive. WoW is not even a phenomena. Everquest and Ultima Online where. As where MUDs before them. WoW just follows on their steps providing the same general formula.

The necessity many players (especially young people) have to feel they are strong/powerful/invincible is what makes these games so rewarding to many. That and the social interaction mostly based on their perceived characters, instead of the real person behind the character.

If a given game provides a progressive rewarding system in which power is the main concern, but also the game is developed around the notion that power establishes an hierarchy in the game, players flock in big numbers. If those rewards are then randomized somehow (Blizzard introduced this old concept to its own games starting with Diablo and made it an art in Diablo II. However MUDs where probably the ones inventing the concept), players stick to the game for much longer than what is probably advisable.

Skill is an element, but not the major one, to achieve a given reward. Persistence is the word. Some objects in Diablo II have "drop rates" so low, they closely match your chances at winning the lottery. And yet that is exactly what drives players to slowly get addicted to the game. The same feeling of... "hope" that makes people play the lottery.

The eyecandy a game may have to offer is not a factor. MUDs are renowned for being text based only and drawing players that dwarf most MMPORGs in numbers. However the game must establish a culture. And this is perhaps the most important reason to addictiveness. If the game is nothing more than rewards, it will fail. But if it is successful in establishing a culture that goes way beyond what the developers can code in any programming language, then you have a winner (for the developers. A nightmare for the parents).

That culture is usually clued in by game elements. It's up to the players to develop it. Programmers can only sit and cross their fingers. Guilds, Clans, certain objects, a layered society, races, allowing the so called "renames" or "alters", providing the skeleton for a in-game economics... all these clue the players to willingly or not create a game culture. Once that is established, the player can definitely become addicted for more (much more) than just a fortnight.

Rewards are fundamental. But without the social mimicking, any game is doomed to failure.

This is just my opinion.