-An interesting stylized aesthetic.
-Memorable, haunting music.
-Stuff that looks like it's glowing without being bloomy (Botanicula, Journey...Actually, things looking like they're made out of light seems to be a popular trend in electronics. I like it.)
-An atmospheric setting. It's hard to define, but it has to feel lonely, sad, and mysterious.
-Exploration rather than combat being the central focus of the game.
-Very little in the way of dialogue or GUI.
-An imaginative fantasy setting that doesn't follow the elf/dwarf/etc. themes of standard Tolkeinesque fantasy (Yahtzee has talked about this a lot, and I've always felt the same way).
-Symbolism is always a plus.
-Ideally a story that involves some personal interpretation.
Yes, I played Journey today. Can you tell?