IMO, one advantage of SPs over MMOs is that you can actually finish the game. And the story actually comes to a conclusion. MMOs can never really tell a good story (or those that I have played at least, don't). It's always get to level uber quickly by killing X monster by X level by Y attack and follow this Z build. In SPs you actually feel that the world will crumble without you, that nothing will be accomplished if you don't act. In MMOs, even if you've built yourself up as the most powerful PvP/PvM character ever in your server, you're as essential to the development of the story as that lvl1 newbie that's killing furry rabbits on the tutorial map. Leave the game for a week and everybody's tons levels higher than you and you have to catch up to be able to party with them. And you never feel any sort of wanting to explore any new location in MMOs, you know they'll be filled by some sort of monster geared for this level and a boss that takes this amount of time to spawn with players camping them.
I really don't get what the fuss about WoW is all about. I played it for a month and multiple times after (my friends were telling me to keep playing) and never really got that hooked to it. I guess WoW is the same as cigarette to me, I've also tried that multiple times and I never really got addicted.