There are a lot of missions intended for level 20 characters- pretty much everything past the Crystal Desert. That means, about 8 storyline missions, along with the EA's and quests that go along with them. There's also the Worlds at War content that was revealed during the April BWE, which was difficult enough to wipe out most pickup groups of level 20 characters within the first 30 seconds.... though more organized groups were generally able to make it a significant way through the areas- I'm sure that these will still be providing a challenge after several thousand hours of playing the game.
The PvP content is pretty much all based around level 20 characters- it's a large part of the endgame, and fighting versus other players tends to have more longevity, as players will adapt and come up with new strategies, so no two games are identical. The PvP portion of GW is very addicting- I've spent hours in tombs runs that seem like only minutes had passed. My adrenaline often gets pumping so hard that I occasionally have to step away from the computer for a few minutes to get my hands to stop shaking....

And that's when I'm just playing a healer.
They've also said that they plan on periodically adding new content to the game between expansions to help provide some fresh stuff to do while you're waiting for the next chapter. And if that's not enough, there's no monthly fee, so if you get tired of GW, you can play another game, and come back when you feel like it, or when they release the next expansion.