Bait barrels, treestands, etc. I am sure are removed after the hunt, but after hunting baits myself, as well as guiding bear hunters, i know for a fact that the bears and other critters that frequent the bait will drag everything not tied down away. The first year i hunted, i had others doing the baiting. They used beef bones as part of the bait, and some of the more active barrels looked like a cattle graveyard from 20 yards out. By the end of the season, bones were spread in a 100 yd radius.
I am not suggesting you won't clean up after yourself, just that by placing things like cardboard on the site, it is going to be alot more difficult to get everything back to the way you found it when you pull out.
On a more personal note, when i hunt, i like to be immersed in nature, and staring at a heap of junk kills the feeling for me.
Not meant as an insult, just observation.