At first I was going to say how long it takes to play.
Not to say all short games are light, but longer play time usually translates to complex gameplay - how much consideration is requires before a player makes a move?
But then again plenty of shorter games still feel quite substantial and "heavy" too, though.
Gameplay complexity would be the main thing I guess. Maybe its how many choices does a player have at each turn? How many ways are there to achieve a win?