Depends on the game though a great game imo pulls you in and make you want to play more and more. Most people don't want to slog through 6 hours of boring content just to get to the good stuff. If we are talking about visual novels then for starters I wouldn't recommend daz because from the results I've seen it's not that impressive.
Daz3d render from the popular game Big Brother
Heres a Blender render on the other hand
Secondly give the game a custom look for its UI. Nothing makes a game feel more generic than seeing a UI in it thats been used in countless other games a person has played before. For comparison look below.
Generic UI from Make Love not Waagh!
Custom UI from Nowhere Girl
Looks better doesn't it? I believe tailoring your game as much as possible to make it feel like a unique experience is key for making a great game.
Good story is rather hard to accomplish I find when it comes to erotica. Writers often either get too focused on sex scenes or they drag out the game longer than its welcome like say Nekopara. Honestly for the quality of writing of that game I would of suggested no more than 30 minutes because the writing just wasn't that good but I've only played the first game so maybe I'm talking out of my ass and its one of those things that gradually gets better throughout the series but I'm gonna hold my breath on that one. Sadly I don't have any examples for what I would consider great writing for erotica at least game wise.
Now finally onto my favorite part. Game mechanics. I'm going to leave vn's out as theres really not a lot to them usually beyond the expected features we've come to know. Lets talk about the goals of the game. I honestly think that games that offer all sex scenes by losing are terrible by design. There's no incentive to actually play the game beyond looking at the cg's or animations if the game doesn't have some sort of compelling narrative going for it and/or the game mechanics are just terrible. You might as well just make a stimulation game at that point. Same thing applies to rpgm walking simulators. Now what makes a great game? Mechanics that feel smooth and actually add to gameplay along with providing a bit of a learning curve. Never have I had so much fun in a porn game until I played If Adventure Time was an Anime. Sure theres little porn in it but the gameplay was solid and I actually had to put in some effort when fighting bosses. I enjoyed Mansion as you actually had to beat the game to obtain all the cg's and the art was wonderful but its game mechanics felt clunky a bit but I could feel the potential for a great game.
Conclusion for a game to be great in my eyes it has to have more going for it than art and sound otherwise you might as well just make an animation and call it a day or a cg set because if a game makes me feel bad when I play it and I question why I'm playing it in the first place I'm just going to find a cg rip of it some where or rip the game myself if I really like the art. Why would I want to play a bad game when I could play so many other great games?