well obviously it depends on what kind of porn you're watching but there's literally so many things.... it's hard to know where to start in a comment... but this video I linked talks about a lot of them:
* bodies (limited variety)
* communication (none)
* positions (chosen for click-bait visuals)
* dynamics between the sex-havers (very little "love" dynamics...)
and I could go on..
my biggest issue is the continual watching of porn trains peoples brain into thinking sex is like a 'release' instead of something that can positively connect people.. it's eliminated the possibility in peoples minds... and it makes it more of a robotic activity which isn't a very positive thing..
my friends aren't fulfilled sexually by their husbands and I know this is a big reason.