When it comes to real porn vs cartoons, I think it helps to consider that you never see a chick in "hentai" who has a hideous face but decent enough boobs or is just willing to do "nastier" stuff.
Women in "hentai" are typically cute to beautiful, taste accounted for.
It's typically a factor of "not my thing" or "bad art" instead of "that chick is hideous" where as in real life porn it is more of a "she is attractive enough while still doing things that I like."
Furthermore, to separate the woman into just objects to carry out acts or fetishes with and dehumanizing them by putting them into a cartoon allows for easier adaptation to fantasies without having to think about the actual person doing the deed such as the mole thing Zed mentioned.
Add to that the fact that with cartoon chicks, you can consistently find girls from series you know or continuing series or stories/pictures and that adds to the familiarity of the girl, therefore for some people this adds a great deal to the experience.
Time ways in to the matter. The average porn picture of a real person takes less time to be bored with than an similar image of a drawn person. This can be attributed to the mere fact they are drawn allows one to study the artistic quality whereas with a real person you automatically know "that is what a shadow looks like, that is what a foot looks like." Add in doujins and other such comic forms and the length of time it takes one to "read" a single image file of a drawn person is considerably higher.
And the fetishes are much easier to find in drawn forms. Simply for the fact that they don't have to find someone who looks like this or has this or is willing to do that. And it's cheaper to replicate.
There are other things to comment on in the differences between the two, buuuut I think you get the picture.
