This can have strange effects on energy sensing spirits, causing them to feel somewhat empty when journeying to the human realm (That is, of course, until they find a wall socket and stick their fingers into it). 
The third and final difference is only really detectable by "Jackson". The reason for this is that the Darkness itself in the Human world is a tad bit different from the Darkness in the Spirit Realm, and 'Jackson" once insulted Human world darkness, it doesn't like him anymore and refuses to even speak to him at the annual Darkness dinner party and mixer. 
Jackson sighed as he walked forward and awaited Mytch's arrival, taking a long breathe and filling his lungs with Human air. He almost wanted to hold his breathe after doing so, for the taste of smog and gasoline that filled the impure Earthen air was enough to disgust even a creature of pollution.