Before Wrath of the Lich King came out, I had a generally negative outlook on the future of World of Warcraft, thinking that its population would slowly decline with time. After having played the expansion for a couple of days, I'm not so sure. It is obvious that new games have an advantage of better use of progressing technologies, and in that they can innovate more. Innovation to World of Warcraft is limited, as total revamps of a game like the NGE of Star Wars Galaxies do more harm than good. So how can WoW compete with those new games? The answer is quality.
I did over a hundred quests (and have the achievement to prove it) in Howling Fjord, and I visited five 5-man dungeons, and I must say the quality of Wrath of the Lich King is excellent. Of course there are still "kill 10 foozles" quests, but not exclusively, and the variety of quests has grown tremendously. I've shot a giant with a cannon from a ship, mated a sea bull with a sea cow, disguised as a wolf, fed a pirate captain to his pet bear, and controlled an abomination to kill monsters by exploding. And that's not all, and all that in just one starting zone, with my next character being able to start in a completely different zone. In dungeon boss fights I fought the mirror images of my whole group, got transformed into a skeleton, had a fight in which I had to move a step after every spell cast, and explored a dungeon in which there were as many bosses as trash pulls. And through 4 days of intensive playing I had not a single crash, nor experienced any bug more serious than a graphics glitch.
I prefer the term quality of execution over the term "polish", but however you call it, Wrath of the Lich King has oodles of it. And if I compare it with the last two major MMORPGs released, Age of Conan and Warhammer Online, I can only say that Wrath of the Lich King wins easily in the quality department. That isn't to say that somebody can't prefer the faster combat of AoC, or the PvP of WAR. WoW remains WoW, and if you prefer a fundamentally different sort of gameplay, WotLK won't deliver that. But at no point in Wrath of the Lich King does one have the impression that one is playing an outdated game.
The only problem that remains for Blizzard is the quantity of content. Wrath of the Lich King has plenty of content, and if you foresee the addition of some already promised content in future patches, there is certainly enough content to play this for a year. But nobody expects the third expansion in a year. With 2 expansions in nearly exactly 4 years, the average right now is 2 years per expansion. And there simply aren't 2 years worth of fresh content in WotLK, even with typical added content patches.
But most people I know are having a lot of fun right now, and that fun will last at least for a couple of months. I'll probably stop playing WoW at some point again, but if a third expansion promises to be as good as Wrath of the Lich King, I'll buy it again. If Blizzard would manage to bring out one expansion like this per year (and I know thats a tall order), they could probably keep me playing their game pretty much permanently. Because the PvE type of gameplay is exactly what I'm looking for, and I don't see any other game around that offer this type of gameplay in this degree of quality. If they keep up this level of quality, there is no reason why World of Warcraft couldn't keep market leadership for several more years.