Re: What do you expect from a adult game? Help me understand!
Any game made, whether it has NSFW content or not, should generally be something that people enjoy playing and want to keep playing until completion (some exceptions are done in interesting ways, but those require a lot more work,skill, and planning.) To me, it feels like a game has failed in some way if I keep playing for something that occurs later on, but am not enjoying what happens in the journey to that point. I admit that my standards for games are pretty high, since I don't consider a game good unless I enjoyed more than half of the time playing it and finished on a positive note (the ending can ruin a game pretty easily, Mass Effect 3 before the updated ending as an example.) The games I've found to be best are ones that I get a desire to play again after I've beaten them, even after 100% completion.
In the end though, a NSFW game should be good enough that you can remove the NSFW content and still have at least a decent game. Kyrieru's games are a good example of that. If the game absolutely needs the NSFW content to be considered playable (or worse, tolerable), you may want to look into other avenues to use the NSFW content instead of making it part of a game. A comic, a CG set, a visual novel, each of those would work to show the NSFW content in a way that will still sell if the content is good. It is possible to have a mediocre or even bad game sell decently if the NSFW content is exceptional, but I'd advise testing the responses and feedback with simple games, comics, or stories to feature the NSFW content and get an idea of where it stands quality and quantity wise. If the game is good enough that you believe it will sell well even without the NSFW content, you may have overshot the goal, but you should still consider whether adding the NSFW content will improve the game or not (how the content is added/introduced can change quite a lot for the game. Collecting pictures of NSFW content scattered throughout the game generally does not leave as good of an impact as earning them through a related portion of the game).
Side note: I don't consider visual novels as games if the player interaction is limited to only choosing one of several options at specific points. They can tell stories in a great way when done right, but it doesn't feel like a game when you read an epic fight scene, and your interaction is limited to choosing whether you say something nice or mean afterwards. Even having cues for directional input a few times would make it a game to me. I have enjoyed several visual novels and still replay some when in the mood, but not for game play.
For the game itself, the developers need to make it with the mentality of a player. The final product should be something the makers themselves would want to buy and play from a complete stranger (there's a bias on buying and playing the game if you know who made it, whether positive or negative). Questioning the game from the viewpoint of a player while making or play testing it is also necessary. Is this area too easy? Is this enemy outright impossible without memorization of patterns or ridiculous reflexes? Am I having fun even during a play test (as a player, not as a developer seeing the fruits of your labor. It is necessary to separate the mentalities for this portion.)?
The suitability of the game for its platform is also important, as some games simply do not work well when put on certain platforms. Games that require complex or multiple controls would generally not work well on mobile devices, for example. Making a game compatible for multiple platforms usually means changing or downgrading some qualities of the game to run on all the supported platforms, or making different "versions" of the game for each platform (like Super Smash Bros for the Nintendo 3DS and Wii U, or Tales of Symphonia: Dawn of the New World for the Wii and PS3. Not really a fair comparison to the end product of the game made with or ignoring these suggestions, but meant to show how much a game can change if put on multiple platforms in regards to graphics and controls.) Focusing on one platform and maximizing the benefits it brings makes the game better and more focused for that platform, but can lead to loss of sales if the game was good enough to have been released for more platforms. Though I don't believe it would be worth it to make games compatible for multiple platforms unless the quality of your previous games on one platform is exceptional enough for several people to look forward to your games.
This was a fairly long post, but I hope it's useful. There is a severe lack of good NSFW games that are worth their price (or even worth paying for. The quality can go that low) especially in English, so good luck.