I can't speak for anyone else, but the main reason why I rarely complete games these days is 'Real Life'; much as my disposable income has disappeared with the arrival of children, so has my disposable time. Years ago I could fritter away hours at a stretch playing Civilisation, but no more. It's very rare that a game comes along these days that I can muster the enthusiasm for to invest time and effort in to complete.
The last game that I played through from beginning to end was "Enslaved: Oddesey To The West", which was an almost perfect title for me; the overall length of the game was quite short (the whole thing was completed over a couple of evenings), the learning curve for the controls was slight and it had a character-led story that I actually wanted to see through to the end. Generally though the sequence goes something like:
- Purchase new game and play for a few evenings when time permits
- Real Life gets in the way and game is not booted for a few weeks
- Arcane control system needs to be relearned
- Plot has become lost in the mists of time
- Cannot be bothered to retrain muscle memory / relearn the plot (such that it is), so game goes back on the shelf
GTA IV is sitting on my hard drive, barely touched - I liked what I played, but I just don't have the time to spend on it. Likewise Left 4 Dead, Mass Effect 2, Arkham Asylum and so on. It took me at least three attempts to finish Bioshock (and I'm really glad that I did), but that's one of the few exceptions. Nowadays I'm finding myself playing more and more 'casual' games (Cut The Rope, Angry Birds, Plants vs. Zombies mostly) rather than 'serious' titles - maybe after the kids leave home and before arthritis fuses my hands into impossible shapes I'll get time to play properly again.