What I think would make for excellent porn is some real plot and scenario. The only thing that I've seen that came close was Caligula by Penthouse. Granted, the true life story of Caligula's reign made this easier to do but surely there can be other, well written scripts, with actors who can act, where hot sex was naturally interwoven into the storyline, where the cinematography was artful and yet felt natural. Also, people who look like the rest of us rather than always having buffed up twinks.
Okay - in short .... Believable and engaging scripts, actors who look like average people, excellent cinematography, and of course .... hot sex scenes. Maybe I am wanting too much from porn?