Excellent article Endo; very well articulated.
I can count the number of games I have finished on one hand. I think this is due to a few factors: repetition, purpose, and time constraints.
When I was a bachelor and college student, I had an inordinate amount of time that I could spend playing games. These days, I'm lucky to get a few hours a night. "Pick-up and play" games like Tetris are fine, but it's difficult to make progress in most 80-hour games an hour or two at a time. A large part of this is context: epic games typically require keeping track of the storyline, your abilities, your next objective, and a slew of other facts. It takes a considerable amount of time -- say 10 to 20 minutes -- to get back up to speed on these things and they are lost between gaming sessions. If for some reason you can't get back to the game for a day or two, you may well forget a large portion of this context. With so much spin-up time, it isn't worth picking back up unless you can devote a few hours in a single span.
For the longer games, I find repetition the most important factor in whether or not I will complete a game. I enjoy the challenge and discovery inherent in learning a new game, which drives me to try every new game I can get my hands on. Once this initial bliss is over, most games leave the player with little to entertain them. The game mechanic doesn't change or offer any new challenges, the "rich, immersive world" often only gives glimpses of innovation every once in a while, and the whole thing ends up becoming a repetitive grind to the end. Completion is great, especially if there's an interesting story behind it, but I won't grind through repetitive gameplay just to see how the story plays out -- that's like peddling an exercise bike to keep the power going to your TV just so you can see how the show ends. As much as games can include interesting narratives, they are still games and still need to offer interesting challenges to the player.
Lastly is the purpose for playing these games. Sometimes I want to delve into a deep story and I have a few hours to devote to it. Other times I just want something to keep me busy for an hour. Sometimes I want a feverish, fast-paced challenge; other times I want to sit back and contemplate my options. All of these purposes are served by different paces and lengths of games.
Of all the games I have played, few have lasted me any significant amount of time. Games with simple but constantly challenging mechanics (due to their multiplayer nature), like CounterStrike and Team Fortress 2, have given me the most lifetime. Even if I haven't played them for months, I can pick them up for an hour and have a good time, and because of this I have played them for months on end. Shorter immersive games such as Portal, Braid, and On The Rain-slick Precipice of Darkness have held me captive to their conclusion mainly because I was able to finish them in only a couple of extended sittings.
Even the better epic games, such as Fallout 3 (which I have loved every minute of), have failed to hold my interest to the end. It's not that I don't enjoy them, just that I can't devote enough time to them to finish them. I usually start off with a couple extended sessions, which get me solidly past the initial-interest period, but then something prevents me from picking up the game again for a few days. When I return, I struggle to pick up the story and remember where I was and what I was doing. After a few cycles of this, I realize I won't have time to really enjoy the game and I leave it until I can.
When I get a new game, I'll often devote a lot of time to it initially, but this isn't something that I can commit to in the long term. If a developer wants me to finish their game, they either have to make it possible within my initial burst of excitement (3-4 fours for 2-3 days) or they have to lower the cost of picking it back up such that I can enjoyably progress through it an hour at a time.
For most games, developers have to start by filling the time from 4-40 hours with something other than repetition of the first 4 hours. For single player games that rely on their game mechanic for this, keeping things new and challenging for 40 hours may be an unreasonable feat, at which point developers should consider gearing their games to be "completed" sooner, rather than trying to drag out a monotonous existence as long as possible.