World of Warcraft has become the dominant paradigm within Western MMO design, for some very obvious reasons. WoW has come to define pretty much everything else in the market. It's become the common touchpoint (again, for very obvious reasons i.e. it's popularity). A shorthand description for any MMO coming out has become "WoW with ________" or "WoW without ________" or even "the WoW-killer".
Of course, that's a gross simplification. But I've seen a lot of people apparently think that only one or two things were responsible for WoW's success and that if their upcoming MMO-of-choice "does <insert system here> right" then it's a shoe-in to beat WoW. I don't see it that way - I see a whole host of factors went into making WoW the market giant that it has become (and remains, at least in the short term). In my opinion, WoW has a success equation that will probably never be met, simply because they aligned so perfectly for WoW that whatever replaces WoW as market leader will have to do so in a way that few see coming.
As a bit of disclosure - I don't actually find WoW that thrilling a game, but respect what it's done for the MMO genre.
The success equation, as I see it:
Time to polish + large development budget + refinement of existing genre conventions + low barriers to entry + established online competitive computer game IP + gameplay that connects to the IP + the Blizzard brand + lots of reasons to keep playing + popularity + good word-of-mouth from beta + luck = WoW's success.
To break it down:
Time to polish: Blizzard made a point of polishing the in-game systems at every opportunity (or at least they've said they did). It wasn't left to the last month to feverishly try to pull things up to scratch - it was a priority from day one. And it shows. I won't pretend WoW is bug-free or perfect, but it tends to outperform its peers in this area.
Large development budget: My understanding is that the cost to launch WoW in North America and South Korea was in the US $65m range. This includes all development costs, server farms, network operations and all customer service infrastructure. In an industry where most MMOs appear to have budgets of between US $15m and US $30m (and may outsource servers, CS et al to a publisher who only wishes to have another MMO as a 'client' and cares very little about how the end player feels about their experience) WoW spent more than double its existing competitors.
Refinement of existing genre conventions: Lots of people say that WoW just took what EQ did and made it better. Although a bit of a cheap shot, it does have a grain of truth to it - WoW devs looked closely at what came before, polished up what worked and threw out what didn't. Combat was sped up. Advancing your character was sped up. Huge death penalties were thrown away. And so on. WoW's innovation was not in designing hundreds of radical new features, but improving the ones the genre had. They chose evolution over revolution.
Low barriers to entry: this has two different aspects - 1) a high end computer wasn't necessary to play WoW, and 2) you could level up quickly and by playing solo. No getting shut out of playing if you didn't meet the system specs (rare in a genre where system creep is a real issue) and no getting shut out by not being able to advance without a group behind you.
Established online competitive computer game IP: The Warcraft series was a well known franchise among gamers, especially those who played online games competitively. As such, it was very easy for players to be aware that the Warcraft MMO was coming and to anticipate playing it. Lots of players had fun in Warcraft games, so they could also reasonably expect to have fun in a Warcraft MMO even if they'd never played a MMO before. Competitive online players, very familiar with Warcraft and Starcraft, looked forward to competiting in a new space within Blizzard's new game. MMO players, who sometimes appear to care less about the IP than they do the MMO part, could be interested that a well-financed, big name MMO was coming out.
"Ahh," you may be thinking, "but don't other games have good, recognisable IPs only to flop after launch?". Yes, which is where the next factor comes in to play.
Gameplay that connects to the IP: Playing WoW fit in with how you'd expect to take part in the Warcraft universe. Warcraft is all about one side battling another - Humans Vs Orcs, and so on. WoW continued this aspect pretty seamlessly - you picked a side and your character fought for that side while increasing in power. Also, while the Warcraft IP did have major characters in it, they were more like representatives of a side, while the player was left to be the 'hero' of the game. WoW continues this to some degree, although (as with all MMOs) being the hero in a world full of 8 million others can stretch the credibility of this somewhat.
Looking at other major IPs, such as The Matrix Online or Star Wars Galaxies, it is clear that the gameplay often didn't connect well with the IP. In The Matrix films, you see the established heroes of Neo and Morpheus taking on Agents and winning - this was suicide for most players to try in MxO. So immediately the player feels reduced in scope and disconnected from what their character 'should' be able to do. Thinking about Star Wars, the films are space operas of lightsaber fights, aerial dogfights and blaster battles. SWG had no player Jedis on launch, no space combat on launch and turned players into canteen dancers or hairdressers - hardly the stuff of space opera. In both these cases, the games had the stylings of the IP, but not the heart of it. WoW launched with the heart of the gameplay linking strongly to the IP.
The Blizzard brand: Let's face it - the Blizzard brand helped WoW quite a bit. Blizzard is associated with high quality games that are fun to play. If Blizzard created WoW, then it follows that it must also be a high quality game that is fun to play; the mantra that "Blizzard doesn't release bad games" is one that is repeatedly echoed over the internet in gaming circles.
Lots of reasons to keep playing: WoW gives the player a relatively easy route to max level, but then offers them a lot of things to do to keep them playing. They can get better loot, do raids, PvP in a number of areas, roll up an alternate character, skill up, buy an epic mount... Blizzard realised (at least at launch) that not everyone wants to acheive the same things and provided a number of options that players could get into. That people were still apparently having fun having reached max level no doubt helped keep players involved.
Popularity: This worked as a snowball effect - players were interested in WoW, told their friends about it, those players got interested in WoW, got them interested in it, etc, etc. WoW hit a critical mass where hordes of players joined because all their friends were playing it.
The worldwide launch also comes into play here - being able to say you've got 8 million players (even if half are in China) makes the game sound like it must be good in order to get that kind of attention.
Good word-of-mouth from beta: Although there were some grumblings from beta about certain things, few MMOs seemed to have the overwhelmingly positive word of mouth coming out of beta that WoW did. Given the strength and importance of word of mouth, I'd suggest that players were swayed by hearing so many good reports about WoW that interest built where it wouldn't have before.
Luck: Yes, WoW got lucky. A large number of factors lined up perfectly to exceed any predicted subscription numbers for WoW. It came along at just the right time and offered just the right mixture to attract a lot of new players into the MMO genre. That said, it also did a lot to make its own luck and grab hold of the opportunity that Blizzard faced.
I don't believe that any upcoming MMO will be able to match WoW's success by duplicating one or two of its systems, or 'fixing' some sort of perceived deficiency. Copying WoW is only going to see players leave any new game since they'd already experienced WoW before. If you believe in the Anna Karenina effect, WoW's success was unique to the combination of factors behind it. Simply duplicating one or two factors - say, the budget and good word or mouth from beta - isn't going to drive player numbers up to the same level because other critical factors are missing.
So what can unseat WoW at the top of the heap? I can only see three things that could - 1) WoW devs shooting themselves in the foot through unpopular choices that see players driven away, 2) lots of niche games splinter the WoW playerbase to numerous other targets so that no one MMO is "the WoW-killer", or 3) some new MMO comes along that succeeds completely on its own terms.
But in any case, trying to out-WoW WoW is a strategy that will never work.