I'd say the big thing that defines a timeless game is the story and atmosphere. Chrono Trigger, for example: fairly mundane RPG gameplay, 16-bit graphics, but

I'd say the big thing that defines a timeless game is the story and atmosphere. Chrono Trigger, for example: fairly mundane RPG gameplay, 16-bit graphics, but the fantastic story has made me replay it several times over the years. Or one of my personal all-time favorite games, Elder Scrolls: Morrowind. It's a deeply flawed game both in terms of gameplay mechanics and technical details (it's shockingly buggy) but the deep, intricate story and well-realized atmosphere make it, in my opinion, much more fun to play than its newer and much more beautiful sibling, Oblivion. I was thinking of re-playing it just yesterday, in fact. Half-Life is another example. System Shock 2 has a deep, creepy atmosphere and engaging story despite looking worse than Half-Life, and it's one of my favorites. Mass Effect is a more recent example--while it's still near the top of the heap graphics-wise, I thought the gameplay was pretty flawed. Despite that the amazing story and characters pretty much guarantee that I'll come back to it in 5 year's time. For me, it's all about the story.