Gaston at Not Addicted has noticed this too. In order to find out why, he compares WoW with some of its predecessors in the MMO genre. Reading through his descriptions of these other games, I wondered what in the world were their designers thinking putting in features that just kept people out instead of drawing them in. Long waiting times between battles? Impossibly long leveling grinds? A mapless terrain with no means of quick travel? It shouldn't take a brilliant game designer to figure out that these would not be popular features.
Blizzard has certainly improved on things a bit. But what really makes for the difference? Gaston says that the reason we keep coming back to WoW is because of "instant gratification." "Most people," he says, "can get a WoW fix in just a couple of hours and usually have some small reward to show for their minimal efforts. Factor in an extremely lax death penalty and you have a slap-happy lollercoaster ride that dishes out free levels like bank lollipops."
Perhaps he's just saying this in comparison to the other games, but I think there's something more in WoW. There's a real sense of story and progress, with dynamic changes and climactic achievements that draw you in and never let you go. For me, WoW is something like a favorite story (like Lord of the Rings) a favorite challenge (like chess) and a favorite coffee house (a space to sit down and spend time with friends) all in one. I certainly don't think of it as a "fix" I can get in just a couple hours.
But who knows? Maybe WoW is just the first game of its sort to come along without glaring design flaws built right in from the beginning?