Given that few people these days finish games, I thought it was worth mentioning that I finished Mafia 2. "Finished" as in played through until the end credits, not 100% of all achievements and collectibles done. I then uninstalled the game and deleted it and the save games from my hard drive, as I am not planning to play it again. As I said before, Mafia 2 for me is an interactive movie, and while I had fun playing it, it wasn't enough to make me play the game again. Personally I found there was too much driving in the game; fortunately I had an XBox 360 controller for my PC, which made driving smoother, but still I preferred the shooting part.
Although I am certainly one of those "aging gamers" CNN talks about in the article I linked to, I also have 10,000+ hours of MMORPG done. So 20 hours isn't too long for me. But I wasn't exactly wishing that Mafia 2 was much longer either. At some point you feel like you've seen all of the city, played around with all of the weapons, and the rest of the game is just repeating what you learned earlier.
I certainly do have left many, many games unfinished. I think I am more motivated to finish an "interactive movie" kind of game than one with a weaker story which goes on and on. For example I never finished Borderlands. I do agree with the observation that one reason for not finishing games is that there are so many of them now. Between free games and Steam sales one doesn't have to be a millionaire to accumulate a huge collection of games. And with a job and family one then easily ends up with too little time for too many games. Finishing a game ends up not being a priority, unless there is a gripping story of which you really want to know the end. Too many games have rather boring stories, and repetitive gameplay, so once you got the impression that you've seen everything interesting, you're more willing to stop playing without having reached the end.