Notwithstanding the fact that the MMO market has increased more than tenfold (in revenue) since 2005, making it much easier to get customers nowadays than it was back then, let's just compare figures then.
Wow sold 240,000 copies (physical boxes only back then) within 24h at launch. Back then, that was a huge hit. Within a week or so, estimations says it had more users than the then-leader, SWG, over 275,000. (http://www.1up.com/news/world-warcraft-hits-big-launch)
Within a year, WoW reached 6 million players, an average increase of 500k per month —roughly 1 ARR coming to WoW each freaking month! Remember, this is 2005, when the whole market was smaller than 5M prior to Blizzard's entrance.
But let's be fair, once again, this was a whole different time, and WoW's increase in subs was in fact the MMO market itself increasing. Pretty much the same happened back in the late 1980's when Nintendo helped expand the video game market to unprecedented highs —in 1990 they owned no less than 90% of the market. No one did that ever since. And no one probably ever will, unless there's a revolution happening in this industry —and even in such a case, for instance with the advent of VR (think Occulus Rift), or later on non-intrusive neural interfaces (controlling games with thoughts just as you would simply move your arm without even thinking of it; yes that is possible and currently the most advanced research sees professional helicopter pilots controlling a flight simulator with their thoughts), it's more likely that the transition would be smooth, and major actors being at the forefront of said transition themselves, progressively eating the newfound customers.
Back to our topic. If you want to compare that to FF XI for instance, here's another chart. It actually shows pretty well that FF and WoW were not competing (no loss for FF XI in 2005), unlike Everquest I&II which pretty much died following WoW's release —and SWG, a bit more slowly, but then they shot themselves in the foot with the NGE.
In 2012, the global Pay-to-Play (P2P) MMO market is estimated at 180 million players. Whole different picture, right? So considering that back in late 2005 it was probably under 9 millions, it means that a 2012 equivalent to WoW would have to reach 120 million players —that's just so unlikely. The biggest P2P in 2012 was still WoW at about 10M.
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Now, let's be really fair and realistic for a second. Let's nuance all of this.
Nintendo went from owning 90% of the market in 1990 to 10% in 2013. WoW tanked as much between 2005 and 2013, but in both cases, that's besides the point. The fact is that most analysts agree that WoW, like Mario, like Star Wars (the original trilogy), like Michael Jackson or the Beatles, like Dallas or the Simpsons, like Harry Potter or Lord of the Rings, are not just "products". They're pop icons, popular wonders, they're members of these select few that reached a level of global awareness beyond reach for 99% of the rest of products. They open new frontiers, set up new horizons, push market boundaries by appealing to a whole new crowd that didn't even know they wanted it. It's unfair to compare WoW to other MMO's because there's more to its success than simple numbers, simple features and designs; there's a kind of commercial and media alchemy that happens a few times per century, not even once per decade in our recent, modern history. It's like comparing the latest pop phenomenon to Michael Jackson… that's just unfair for the former whose career is just beginning and totally unlikely to reach anything close to the King.
So it's rather pointless to compare figures, and it's rather pointless to try to find "within" the products the reasons explaining the magnitude of their success. You may find excellent design/features/directing/production, and surely these wonders are often considered masterpieces from a strictly artistic standpoint, and they set new standards for their whole trade/industry, but that's far from being the whole story. It's been a long time now, several years, since anyone in the P2P MMO industry tried to even aim at WoW-like numbers, because it's just indecent and disheartening —doomed to fail in this respect. WoW will die because of WoW (or, more likely, because Blizzard itself will launch a new MMO someday), certainly not because of the competition. Just like Nintendo saw its market share shrink during the 1990's, a bit thanks to the Genesis in the US for instance, or other competitors in Japan, and eventually of course the beginning of the PlayStation era; but overall more so because the world of gaming had changed, and even they [Nintendo] themselves couldn't just retain all these players with their different tastes and whatnot (plus, they kinda failed the optical transition with the N64, however inventing no less than the analog stick). But as Iwata (Nintendo CEO) often said, I believe very sincerely: "Nintendo is glad that PlayStation and Xbox are doing well because it means that the market is growing stronger and that's more players overall, more potential customers for Nintendo."
Honestly, I played WoW beta/vanilla, and I'm positive the experience was great but it was not as good as more recent MMO's as far as leveling and early endgame is concerned; but that's to be expected a decade after. If one thing, it would be worrying if the comparison even held water between decade-apart titles.
As we speak, what matters more is the retention rate: WoW will always be higher, but its real strength is that it remained quite stable for such a long time, and that's where Blizzard really deserves credit: they did better with Burning Crusade, they did better with Wrath of the Lich King, and even if Cataclysm or Mists of Pandaria were able to bring back players when they were released, only to see 10-20% leaving afterwards, it's still quite the achievement. Few other MMO's can claim to have such a high retention rate (or "balanced turnover" between those leaving and those coming back, more specifically, among the ~30-40 million players that own a WoW account). That's where other titles can compete with WoW, albeit in a different "weight category" so to speak: if they see even a mild increase in player base, they're gold. That's where EVE shines for instance, regardless of its much lower player base. That's where profits come from, because obviously the whole development is scaled to the existing player base (if it increases, you make money; if it shrinks, you're unlikely to be able to sustain new content as frequently or for as long as you'd wish). That's the reason behind most of these titles going F2P, simply to sustain themselves and remain profitable (going F2P is like going low-cost: you yield much less per customer, in average ~$3 compared to $10-12 in P2P, but you trade that off for triple the player base or even more, and retention is less of an issue when coming back is essentially costless for the players, thus much easier to pull off with a nicely targeted PR).
What will be interesting, though, is to compare how ARR fares after a year: will it be as stable as FF XI in its time? Or will it see a massive increase? Will other new titles such as ESO or Wildstar hurt it? Will the next WoW expansion follow the same "hype-then-downward-trend" as the two previous ones, therefore hurting everyone in the market at first but then slowing leaking players back to other titles? There's a high "hopping rate" between MMO's, them attracting a rather specific crowd, therefore that WoW does well is, as Nintendo's Iwata would put it, "good for the MMO market": that's a huge player pool from which other MMO's can potentially attract players.
Even more interesting, in our specific ARR case, will be the first expansion, in 15-21 months based on Yoshi's estimations. Will it pack as much content as a WoW expansion for instance? Will it bring numbers much higher, in a clear increasing trend as BC or WotLK did back in 2007 and 2009, or as EVE did during its whole history? Or will it simply serve as a come back / stay here incentive for existing and previous ARR subscribers, much like FF XI saw its expansions not particularly increasing its player base? These are interesting questions.
If history is any guide, what WoW did great is:
1. Never stop rolling out new meaningful content
2. Constantly reflecting on its own design and changing it over and over again until it managed to please most kinds of players (from hardcore raiders to the most casuals passing by moderately active ones) —this is still an ongoing process, 10 years after it launched, and its many iterations saw an overhaul, sooner or later, of pretty much every single system of the game, including class design and such. That's bold, and arguably a form of mild innovation, and it kept players engaged because there was always something new to try.
3. Excellent marketing/research and following-up of its player base (including customer support, polling its own players for feedback, and awareness of its competitors).
Also, last remark: one should not compare the "whole" of WoW to other games; the only relevant content at any time is that of the current expansion: there may be 20+ raids but people don't come back or stay because of the older, obsolete ones. So to compare apples with apples, there are only a maximum of 5 raids or so at any time in WoW for instance (usually 2 when an expansion is launched, 5 or 6 when it ends). Which is why I suggest we wait to compare FF XIV's first expansion with Warlords of Draenor for instance, that is relevant: regardless of what's existing before their release (of course, WoW will have more classes/specs for example), what will these expansion bring in terms of content? Will it be comparable quality-wise? What will they do (increase, decrease, no change…) to the respective active player base of these two games?
My (long, sorry) 2 cts.
EDIT: I have a fairly educated opinion on what's going to happen with ARR, but I won't risk predictions here. Let's just say I hope, for SE's sake, that they're ready to really improve what needs improvement, and listen to players at large beyond their own fan base.