A few weeks back (okay, a month back, but I started writing this two weeks ago and just got distracted...), this thing happened. This Diablo III thing.
I remember the excitement of playing the original Diablo "back in the day." Long hours spent navigating the catacombs of Tristram, slaying monsters and earning fat loots for my trouble. I remember dark corridors, weakly illuminated by flickering lights (and my own torch). I remember the soundtrack - music that very effectively added to the intensity of the game's artistic design - overlaid with occasional distant screams and the sound of gibbering hellions waiting for their opportunity to strike. I remember the pain of the Hellfire expansion, but playing it nonetheless, because - p'sh! - moar Diablo!
I played through Diablo II several times, and I remember enjoying the Druid and Amazon classes most. I remember the thrill of seeing the Secret Cow Level, and the agony of being slain by hordes of polearm-wielding bovines. I even remember the names of some of the bows I found (Magewrath and Windforce come to mind), and how excited I was to be able to use them in my quest to end Baal and Mephisto.
In short, this game series has provided a wealth of memories and hours upon hours upon even further hours of gameplay that kept me glued to my seat through multiple replays. The reason has always been the beautiful combination present within the game of single- and multiplayer potential.
I enjoy being able to sit down and relax playing video games (as you've no doubt gathered already), but there's more to me enjoying a game than just a good story or good gameplay. I can get those from a book or angry birds. What keeps me coming back to a game for years (see also: WoW subscriber from launch until earlier this year) is a solid system for encouraging and rewarding group play, while also allowing a rewarding single-player aspect, for times when I'm either not feeling up to socializing or when I can't find a group to run with.
This, to me, is what Blizzard got right with Diablo III. There are a ton of people complaining on the greater interspheres1 about all the things that are lacking, but what will keep me interested in this game for ages is that I can play all by myself and make progress, but when I want to play with others and have the opportunity, the game gets out of the way and makes it simple to do so.
Anymore, that's my criteria for a successful game. Single player-only games can be amazing, but I will quickly lose interest and move on to something that I can share with other people, not as an amusing anecdote after-the-fact, but as a shared experience.
It's my little slice of social interaction that allows me to remain a cave-dwelling hermit the whole time.
1 - Said complaints run the gamut from "the story is two-dimensional", "the real-money auction house is a moneygrab/sanctioned cheating/the end of the franchise (take your pick)", "always-connected DRM sucks" all the way to "I can't kill things the way I want to, so make the game easier" -