When it comes to the factors that are important, I think these are the most relevant from most important to least important:
1) Does it have the fetishes/kinks I like? By far the most important for obvious reasons, when it comes to porn games. Unfortunately the one where I get disappointed most often, because the lesbian tag is used far too lenient here.
2) Does it have good artwork? OK, that one is obvious too. The characters need to look hot to jerk off to their image.
3) Does ist have characters and a story that deserve being called characters and a story? I just wanted some hot scenes, I'd find them on YouPorn as well. I'm not as demanding when it comes the story of porn games as I am when it comes to the story of regular games, but I do think, that characters need a background and some development.
4) Does it not have too much grind or is this grind avoidable? In a porn game I don't want to fight 500 zombies and play 200 mini games to open another character's bra. I'm no friend of grind. If a porn games has grind, I'll gladly cheat to avoid it.
5) Does it already have some content? I'm not extremely picky here, I don't demand finished games, but they should already have one or the other hot scene in the versions available.
6) Does it "work"? Does a game run in a away that is enjoyable. I don't mind the occassional error message, but it shouldn't crash every other minute.