It's an element that may have worked in DX, but that doesn't make it crucial in an exploration game. You could just as well focus the gameplay on other aspects of exploration and make it just as enticing. For example, in this game I'm making for the PS3, there are toilets hidden throughout the gameworld. The player character can only go through a certain amount of time before his/her bladder/bowels give out, which then leads to interesting gameplay situations. As such, the player is encouraged to explore the environment, and search for the toilets of the world. Some of them are even very well-concealed. Furthermore, we even added time as an incentive to explore on a deadline, given that your bladder/bowels only have finite space. Now see, in our secret PS3 game, time is also another important resource in exploration, much as health was in DX. As developers, we thought it was a novel and different approach to exploration, rather than just hiding health kits or star maps like in every game ever made.