Funny that this thread crops up. I was a long-time player of WoW, having a few characters on the Alliance side, but I stopped playing early last month as I was getting bored with what being level 70 had to offer me.
About a week ago I re-installed Warcraft III and I was instantly reminded of just how good that game was. Sure, some would say Starcraft is better, but in terms of the story line, it is an absolute gem. The loading screens all have maps on them and I've often found myself looking at the locations and saying to myself, "I've been there, and there, I remember that" and can immediately associate everything with a picture in my head. Add to that the fact that running around in the game world is pretty close to navigating the lands in the World of Warcraft, and the consistency is fantastic. So much so, that I'm actually finding myself gravitating toward re-activating my old WoW account, but rolling a new character on the Horde side to experience the game and story line from that angle instead.
To all those that are saying WoW doesn't have a story line, you can't be paying much attention to the lore and stories being told through the quest chains that you do when playing. Right from the very beginning, there are stories that describe the plight of the Dwarves and Gnomes, stories that interlink with the other races, stories about how the world has come to be the way it is, what happened after the wars, quest chains like "The Missing Diplomat" (alliance side), Van Cleef, The Scarlet Monastery, etc. Everything has had a part to play in the overall story, and they all in turn delve deep into the lore of the game. WoW Wiki is probably the best site for more information on all the lore to be found in the game, and has back stories for almost all the major characters and scenarios that you encounter when playing. Unfortunately most people see WoW as a grindfest, which it is if you're onto your 5th or 6th character, but if you pay attention to the quests, and gather an understanding of just exactly why you're doing what you're doing, it's a lot less monotonous. It's all too easy to just ignore the quest text, but WoW itself is just one big story, and it's brilliantly done. If you take away the boring PvP and silly end-game encounters, and spend some time learning about the world and why it is how it is, it's very easy to appreciate just how much time and effort Blizzard have put into their franchise.