Looks are required to draw the attention. Gameplay helps to keep it, but most of all it has to fire a player's imagination - whether they live vicariously through the eyes of a weenie turned supersoldier or are somewhere so weird that it invites exploration.
That third part is the most important. Books, movies, films, music, games - the ones that people go back to are the ones that capture their imagination.
Capturing the imagination can be as simple as appealing to the right hormones for the right age group, so it's actually not a necessity for the game itself to be original. In fact, if the player has to spend a lot of time familiarizing themselves with it, it destroys the immersion.
This is why "me-too" FPS games and warrior hotties swinging swords as big as cars and wearing chainmail bikinis continue to dominate the shelves despite an utter lack of anything new or cool about them in the last ten years...