Blizzard has taken a bit of flak from the Diablo 3 fan community since the game's release over the lack of proper endgame content, an issue the developer admits that it is aware of and working to address. Thoughts are already turning in that direction, though work on some of the game's in-progress updates will have to be completed first, a statement from Blizzard community manager Bashiok on the Battle.net forums confirms.
A single playthrough of Diablo 3 on its starting difficulty will get most players roughly halfway through the 60-level cap, with subsequent playthroughs on the higher difficulty yielding more experience levels as well as higher-end loot. Bashiok nods to that and to the game's upcoming PvP arenas update (v1.1) as offering solid replay value, but admits that these don't solve the problem. Blizzard will address the issue in time, he said, but it's not going to happen until after v1.1 is released.
The statement came as a response on a user-created forum thread asking what sort of plans there are in the future for endgame content. The poster makes a very valid argument that "Act 4 is currently pointless due to the small number of elites and the the drop rates being identical to Act 3," which renders the earlier act "superior" in his mind. Bashiok acknowledges the shortcoming in his straightforward and informative statement, the whole of which you can read below:
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.
We're working toward 1.0.4, which we're really trying to pack with as many fixes and changes we can to help you guys out (and we'll have a bunch of articles posted with all the details as we get closer), and we're of course working on 1.1 with PvP arenas. I think both those patches will do a lot to give people things to do, and get them excited about playing, but they're not going to be a real end-game solution, at least not what we would expect out of a proper end-game. We have some ideas for progression systems, but honestly it's a huge feature if we want to try to do it right, and not something we could envision being possible until well after 1.1 which it itself still a ways out.