Blizzard developer 'Bashiok' has made a startling admission on the official Diablo III forums by stating that the hunt for items, the mythical "end game", in D3 may not be enough. He goes on to further state that devs have much in store for players with the next pair of patches.
We recognize that the item hunt is just not enough for a long-term sustainable end-game. There are still tons of people playing every day and week, and playing a lot, but eventually they're going to run out of stuff to do (if they haven't already). Killing enemies and finding items is a lot of fun, and we think we have a lot of the systems surrounding that right, or at least on the right path with a few corrections and tweaks. But honestly Diablo III is not World of Warcraft. We aren't going to be able to pump out tons of new systems and content every couple months. There needs to be something else that keeps people engaged, and we know it's not there right now.
Read the post and participate in the discussion on the Diablo 3 forum.