You are immune to gravity or gain some kind of buoyancy, hence not plummeting through the ground. You also gain some self locomotion, so as not to worry about friction, (though many intangibles can float or explicitly fly). You can still interact with air normally allowing you to breathe (if you need to breathe), hear and speak.
Furthermore, an Intangible Man is still anchored relative to the Earth. Remembering that Earth whizzes around the Sun at a frankly irresponsible speed, anyone who can walk-through-anything should find themselves suspended in space a few seconds after their powers manifest, watching the Earth slowly but surely arcing away from them...
- That is if you consider the sun the center of the universe, or anchor on which reality rests. Otherwise, you may find yourself speed from the solar system and/or galaxy. Maybe the frame of reference is voluntary? After all, if these can work on space ships when they turn or other planets...
- One excuse is that the Intangible Man is presumably moving at the same speed the Earth is before becoming intangible. Therefore, unless some force changes his velocity or the velocity of the Earth/the Sun/the galaxy/whatever, he willseem anchored to the Earth. If he is immune to gravity, however, then he will merely travel in a straight path at the velocity of the Earth at the current moment. As the Earth orbits around the Sun, it's velocity constantly changes, though the Intangible Man's would not
- It is worth noting that intangible characters often become so by transforming into a sentient mass of energy, and most types of energy are able to move through the atmosphere without shooting off into space, albeit not consciously. See Shapeshifting above for the problems this presents on its own.
Alternatively, you remain affected by gravity but your intangibility makes an inexplicable exception with the ground so you can just walk on it normally instead of falling through.