This is a pretty interesting discussion. When is a game finished?
In my book, I have 'played a game' or 'finished' a game when I have mastered its mechanics. When I can sit down and require zero thought or real effort at playing a game, I feel I have mastered its mechanics. For some games this happens pretty quickly and others take a LONG time to get there. It's not about finishing a race in a racing game (or ending first), it's about getting under the game's skin. After that, the only real achievement in said game is a time investment, and I don't see much merit or value in that unless I really, REALLY love the game.
With the above in mind: I finished World of Warcraft vanilla and TBC - I have completed all the content in there, including every raid and boss battle. I mastered all the mechanics of all those fights, hell I could dream them, today.
I also finished Guild Wars 2 vanilla until the Fractals were introduced, going by the same principle.
You see even for an MMO, after you have mastered all mechanics and fights, there is nothing new to see apart from PvP and some items and other stuff on the side. The mindless farm then commences and repetitiveness sets in. I consider that finished.
This idea of mastering mechanics works great for me. Games that focus heavily on narrative and then offer a shitty storyline that lacks quality, I can master the mechanics of the game and be done with it. I do this, if the narrative of a game isn't to my liking I just drop it. I've dropped many games just short of the final mission like this too, but I do tend to pick them up again *much* later, starting a new game entirely and doing everything right that time. I did this with Metro 2033, played 3/4th of the game, dropped it, and finally finished it when Redux got released.
I'm not an achievement/trophy/100% hunter at all, I think that is a massive waste of time. It falls right in line with mindless zerging or farming for me - boring, repetitive, button mash-fests. This isn't always bad, but after a bunch of MMO's I think I am completely past that unless the farming itself is great fun.
Going by my own metric I have finished about 85% of the games I bought. Going by the accepted metric of finishing storylines / main campaigns, I probably finished about 10%. The Witcher 3 has also fallen into my dark pit of unfinished narratives, but I am so tired of the simple combat mechanics and I've seen every zone in the game.
