I'm a little compulsive about finishing every quest in a zone. Ok, ok, I'm certifiably compulsive about it. I love exploring all the various corners of a zone and learning their dangers and rewards. Because of this, I barely got through half of the Outlands solo content before I hit L70. After thoroughly enjoying Hellfire Peninsula, Zangarmarsh, Terokkar Forest and having just a taste of Nagrand and Blade's Edge, I hit the level cap. Why should I continue?
Starryknight of the Executus server has an excellent answer: completing every Outland quest and running through every dungeon will net you the 5,000 gold for your epic mount, gain the necessary reputation to access later parts of the game and get you keyed for both the Heroic dungeons and Karahzhan. Essentially, you will be ready to jump into the raiding game, if you so desire.
To achieve this, he wrote up a general guide on what to do in every Outland zone with links to more specific information about the quest chains for each area. For me, this is what I needed to push on and continue exploring the wonders that Warcraft still has to offer for people already at the level cap. Even if you don't intend to raid, the goal of buying an epic mount is reward enough. Check it out, it may be what you are looking for to refocus your goals in the game if you find your motivation has flagged.