Diablo 3 patch 1.05 is due to arrive fairly soon. It has all kinds of interesting changes in it, the most notable of which is the addition of a real endgame targetted at players who already have extremely powerful gear. This comes in two parts: The first is the ability to set Monster Power to make the enemies much tougher than normal and to increase rewards. The second is a rehash of Diablo 2's Uber Tristram where you farm powerful enemies over and over to gain access to a extremely difficult fight with unique rewards.
I think that Blizzard is doing some good work here but unfortunately there is one key thing missing. People will be very happy with an endgame that gives them a goal to work towards and something concrete to build on once they reach level 60. Even if an individual player never gets to try to fight the Uber encounter themselves they do enjoy the idea that there are things to reach for and nobody can deny that the Uber encounters set to maximum Monster Power are very challenging and require excellent gear and substantial skill. There are lots of other small additions to the game here and there which all seem good so I cannot complain about anything that is actually in the patch notes.
The thing they are missing from this patch, which I complain about every patch, is communication. We have all the downsides of online play like rubberbanding, disconnects, maintenance, forced internet connection, etc. but we still lack decent communication which is by far the single most important upside. I don't know if adding in chat channels or guilds at this point would actually get my community of friends back into the game or not but I do know for sure that without it the chance of us getting back in and consequently pulling other players into the game is zero.
I suppose this isn't the case for everyone. There is a big community of people still playing CiV out there and the game itself is single player but a lot of people do spend a ton of time communicating on various forums. Modders and people who use mods were a huge reason I played as much CiV as I did because I could chat and interact with them to get feedback, give opinions, and just pass the time. There are people and games that get played a ton without any sort of community at all but they are tiny and insignificant compared to the social games. Just consider sports bars, which people go to for games but which have nothing *but* the social element and we can see how powerful the attraction to talking about games is.
The easier it is to build a community around a game the more successful that game will be. Companies that successfully leverage that will make a ton more money that companies that simply produce another good single player game.
Pictures from Blizzard at: http://us.battle.net/d3/en/blog/7597724/New_Event_The_Infernal_Machine-10_15_2012