WOW just announced that it has gone down to 5.6 million subs. This is a far cry from its peak at 12.5 million, but it certainly isn't unexpected. WOW subs have been on a relatively smooth curve throughout WOW's life just as nearly all MMOs are and it is slowly losing subs with time in a predictable way. There are spikes of course, at the beginning of new expansions, but nothing seems to be able to budge the basic curve that continues to emerge.
People have this remarkable tendency to assume that whatever things irritates them most at the moment is responsible for this loss - whether it be pvp imbalance, their favourite class sucking, too much homogenization, whatever. I just read a big chunk of a thread about how the latest structure for raiding is killing WOW, specifically that restricting the highest difficulty of raiding (Mythic) to 20 person groups is a problem. That theory is silly of course because while there are a few 10 person guilds out there who actively disbanded because they want to do mythic difficulty and can't recruit up, most of those guilds were headed out anyway. Those people could have just done heroic mode, or joined another guild wanting to do mythic, but neither would suffice. Pretty clearly they just didn't have enough people that really wanted to play.
My old guild, OGT, was like that. We got a lot of stuff done back in the old days of WOW's maximum popularity but eventually things fell apart when people just didn't want to play enough. In Burning Crusade we put up with the hassles of recruiting and dealing with buffoons and got our guild up to 25 person strength despite starting with 10. We could have done the same again had their been the will to do it, but the will was not there. We didn't want to play WOW badly enough to make it work. That doesn't mean WOW sucked when we stopped raiding, nor that it sucks now, it just means that running a high performance group with a lot of people is a lot of work and weren't willing to do that anymore.
WOW is still a lot of fun. By all accounts the last two raids are among the best raids ever in terms of top end challenge for the best players. There are a bajillion things to do that weren't there in the beginning like the brawler's guild, pet battles, and toy collection. No matter what Blizzard does the same curve is going to apply and subs will continue to slide, slowly trending towards zero.
There are a lot of years before it finally dies though, pet theories about what will be 'the WOW killer' notwithstanding.
And hey, they are about to announce the next expansion. More things to kill! More stuff to collect!