Raid instances aside, you might have missed certain instances if you haven't run up a dungeon-runner since that content was popular. Or maybe you haven't looped back to catch the updated version of an instance you ran many a time back in the day. Perhaps you're just not into PvP, so those areas remain a mystery to you.
For most players, I'd venture to say that completing all the leveling content comes down to a question of faction. Some of us just don't play the other side, or we haven't played it since we were in a previous guild or on another realm. Even those of us who have alts on both sides can sometimes forget little details. You might have run characters through every last zone quest line, but perhaps you've only done scenarios with your current main from one faction's perspective. ("Oops," muttered the annoyed completionist.)
Have you managed to work your way through all of WoW's zone and non-raid instance content for both Alliance and Horde storylines? What chunks of content have slipped past your grasp? Do you have a plan for filling in the gaps, or are you fine not having done and seen every last bit of scenery along the way?