To skip all those hater and shouters that wouldnt see a good game if it hit them right in their crotch...
Is WoW a good and popular game? Yes it is. You may not like it personally, and yeah, 11 million people doesnt prove something is good, especially when it comes to movies or music, but its hard to find any real flaws behind WoWs design.
I get it. You dont like MMOs, you dont like grind, you odnt like end game, you dont like whatever else the genre offers - dont play it, but dont judge it either based only on your likes and dislikes. I dont play WoW anymore either, I found it getting to repetitive and too easy while chopping off too much of the fun things in favor of access and casual gaming style. But it doesnt change the basic fact.
WoW is a good, well made, polished and pretty much perfected example of what some like to call EQ style MMO. It has plenty of content, variety of things you can do, solid world, relatively pleasant to the eye visuals and not bad learning curve.
Its easy enough to let anyone play it, while the highend game does requires at least common sense, knowledge of your class and ability to work with team instead just mashing buttons (emphasis on high because apparently, even with all the easy mode everyone complains about only ~5% of playerbase does full clears).
It did offer a little bit for everyone, from solo content, daily grinding, mindless monster hunting, through casual and more competitive pvp (balance issues aside), group content to take on with friends to end game for the more agitated guilds.
As far as current MMOs goes, it achieved everything it wanted to achieve, but there is no such things as perfect game, everyone likes something different. It just did it well enough in right time, giving a massive prod to the whole MMO market.