Yes there is a single answer to why WoW made it big.

battle.net

Blizzard without Battle.net is but a mediorce game company producing mediorce games. nothing groundbreaking in terms of technology just fun, easy, hacknslash, zergfest games that had a social platform and thats why I say without doubt if battle.net did not form the foundation early on for Blizzard to promote their games they would of went the way of westwood studios along time ago.

Battle.net would explain WC3 and others that actually use it,

But not WoW.

Also,alot of companies use macs.
With Blizzard's games being Mac and Pc compatible,it gave more people the ability to waste huge amounts of company time while they are supposed to be working,thus forming a solid base of hardcore player's able to indulge in wow to their heart's content at work and at home.
No doubt this isn't the main reason wow made it so big but definitely was a step in the ladder imo. 8)

Theres one of these popping up every week, each one with more generic and less insightful reasons than the next. /careface

(And yeah, I read the entire thing.)

you can point out battle.net as being the pillar, but personally i believe that there is one thing that sets blizzard apart from other companies as being a good game company, "pride" or how ever you want to call it. they make a product and they make it good, they don't have to worry about a publishing company yelling at them to get it golden now or never, so they truly do make a game golden when it is golden. yes battle.net is a large positive to them, but is it not just a product of there attempts to make a game everyone will like.

i can guarantee that anyone who has played more then just WoW in there life can think of at least one game where they felt it should have not been released yet or the company ignored it right after release. i personally avoid EA titles now, and i will not be buying another call of duty product as activistion iw is going the route of get it "golden now or never" (hopefully joining with blizzard under a new publishing umbrella will get them out of the hole)

it comes down to the barber philosophy, "your only as good as your last cut"

The author has it right about fun; the game is a lot of fun, with a lot of interjected humor that can cross the nerd barrier. But what ultimately drives it is that enough nerds got on board to bring in their non-nerd friends & co-workers thru tale or witness of their exploits.

What I think really drove this is (no particular order):
1) Very rich story & questing experience, the average player cannot finish "the book" of quests in the game thus can play for a long time without repetition.
2) Reward based PvP, factors more players will play a reward based vs. penalty-based game (no one likes backsliding, or losing all their "stuff").
3) Graphical sophistication and beauty without being overly cluttered.
4) Clear departure from reality (escape), with enough reality basis to be comfortable (mostly adheres to real world physics etc).
5) Compared to any other game of several I've played is plagued by few bugs & most are fixed. Given the newness of many gamers, they won't recall 5-8 years ago games that released but were unplayable for months with little or no bug fixes.
6) Extensive support in terms of customization - the game was designed to make mods easy to produce, and most mods make the game easier to play (in fact often a mod actually shores up a deficiency in the game interface, noting that many mods functions have been incorporated into the game itself).
7) Reasonable level of complexity in characters, though largely thru gear in this case. Characters themselves are simple. This is a pro/con item really. It is a pro for the un-initiated that make up a legion of users, but not such selling point for more hard core gamers. Other factors lure the hardcore, but this caters to the casual.

The game still does have its weaknesses.

1) Travel times/Down Time
a) Travel time (flight etc) was worse, they have slowly streamlined it somewhat, but if I get on a bird in Auberdine to fly to Gadgetzan I may as well get in the car & go to the store, I might have arrived by the time I return. That is *boring* when you are sitting in front of the screen (its neat the first few times to view the game world from, afterward...).
b) While initially one can jump right in, the author polishes over the hassle of end game participation. Forced grouping to gather a platoon of (competent!) players to overcome an obstacle is the undoing of many players reaching end game content. Even grouping for non-endgame instances becomes a chore once the crush has mostly leveled thru to end game content. Recent daily quests have somewhat improved this, but inexperienced players are still on the shaft as they get kicked from groups - there is no direct reward for helping them learn or accomplish their goals & only friends/guild mates would do it for free. The suggestion has been floated to give a partial reward for assisting in the completion of a quest you have already completed thus giving financial incentive for someone to help NewbXn00b complete their Van Cleef run.
c) Inventory blues - Optimizing a character, managing the very limited pack space etc are not terribly fun. Quest items clog one's bank; several types of equipment are necessary to participate in the spectrum of activities (raid, PvP etc) so that all the "Fun" items (Murlock pets for example) have to be deleted by most players. Example, I have a PvP set, a PvE set, and 2 resistance sets of gear (& 2 more partials) & Off-Spec sets for some or all of those (PvP/PvWE). That alone occupies nearly 1/3 of my pack & bank space and I have the max capacity. The Key Ring they added a while back helped this somewhat. They need to take the next step and have a separate space for quest items. The player that went thru a several hour ordeal getting a group, crawling a dungeon to get the quest item at the end does not log off pleased with their experience when they discover they did not get said item because their packs were full. Any serious business analysis would reveal it has no benefit to the game to require them to repeat that process of 100s that they have to do. A "bottomless" quest item bag solves that, there are only 25 quests they players can hold, and a few dozen bound collectable items (Spirit Shards for instance).

2) Static End game - Most casual players don't hit this wall. But there is little day to day changes the players make on the world, and for those that do actually complete the quests & instances there is little else new till the next patch or expansion. Despite being based on an RTS (real time strategy) game, we don't build barracks; we don't raze the other side's cities. There are short-term dynamic elements (taking Halaa for example) but the RTS basis is really absent from the game. With a 10 million subscriber base using them toward a more dynamic end game would be smart leveraging of a monster resource.

3) End game bias - While the end content is static, it also incorporates a bias. The "King of the Hill" effect where people that got the high gear first can put those who have not under their thumb. Success is largely dictated by gear. The arena combat system is badly broken with the selling of arena points, personal rating requirements etc & profession unique advantages. Max geared/ranked players buy low ranked teams and level the team rating up to sell for gold - they skew the game for everyone else low/mid on the ladder by handing out defeats from an opponent they shouldn't have to face unless they won dozens of games to get to the top ranks. Personal rated items create a pool effect, once so many players make that mark, they create a barrier of superiorly geared players that effectively lock out the rest of the player base (people that can't play every week, don't get (buy) as many points each week etc). Also noted, the Enchanting and Jewelcrafting professions appear to provide a significant advantage when you troll the top ranked teams and see 4 out of 5 players have enchanted rings & creator only gems in their gear. They need to remove the bind on pickup effect from these things if there is any intention of making the arena system a fair competition of player skill and not a gear check mismatch. While arenas are but a part of end game activity, the gear provided from them dictates success in the rest of the competitions one encounters in the game world over resources, quests etc. I know many casual gamers for whom this is really souring their experience.

Overall it is obvious WoW is a very polished game. Noting the areas of success & strong points will be essential to making a successful competing product. Figuring out the week points, exploiting them and providing game appeal on that basis is also essential for anyone wishing to compete; that all predicates on Blizzard refusing to address those flaws. If the game's clients are largely satisfied, you can release a better game and most won't come because they won't have reason to leave this one to try it. Success on someone else's part will be dependant on complacency on Blizzard’s.

No less insightful than the other end of the spectrum. And there's probably about the same number of them.