Well, I am such a slow and poor player that I doubt I will ever need that feature... I can remember pretty well all the games I've completed...

I wonder, since the website is already able to define a separate section of the shelf and move all the hidden games into it, how hard would it be to integrate other sections (e.g. like a "Completed" one for the OP and everyone else who'd find it useful)?

people here buy stuff so often they don't even have time to play it so marking finished games might be a bit pointless :D

I never finish a game.

I try to finish every game that I start. Think I'm successful at about 80% of them. Normally I just quit when I totally don't like the gameplay (or some core elements of it), when I can't make any progress, because the game is too difficult or when there are any game breaking bugs or technical issues.