One of the great joys of WoW is how couple-friendly it is. My wife and I are suckers for some cooperative multiplayer, and we haven’t looked back since we set up her account. We play together all the time, doing dailies, farming mobs, instancing and raiding (although we’re learning that being new parents isn’t really conducive to raiding, so that’s trailing off). Also, leveling alts.
We have some experience leveling a number of duos.