I am a pervert. I'll leave my preferences at that.
Something I always found helpful for lemons was when writing, to leave focus on one specific body part or action per paragarph, wax a bit of poetic, but generally avoid words like "member", "penis", "vagina", etc., directly referring to those body parts runs a risk of triggering a readers personal filters and emotional contexts for those words, some people find the word penis too clinical for a scene of passionate sex, but others find words like member to be too overused/silly. To my mild surprise, the breasts and nipples don't seem to have this problem, though the word "boobs" is generally considered too silly for passionate scenes.
While the usual things about good writing are all required, good pacing, flowing diction, in character, easy to visualize, etc. are all important, I found that word choice and ease of visualization were more important than anything else. Good pacing is also high up there, but can be let slide if it's HAWT, and people tend to not mind how in-character people are when they're reading about their SEXORZ. Being vague is your friend, as letting the person visualize things as they think is hottest allows the most powerfully erogenous organ to do its job, the brain.
These are general trends obviously, but something I've noticed in my readings of lemons and their comments/reviews as well as my own forays into them in different fandoms.