I didn't play Karryn's prison, because I don't like the giant-tit-stuff so much ^^ Maybe I should give it a try, I already noticed that it is quite popular.
I think the difference between a hentai and a game like third crisis is that a game always has a better immersion. And immersion is the key element of a role playing game, or what is your definition of a RPG?
The base of a good rpg is the progression of the main character for exemple through skill and stat evolution regarding the player preference as well as interesting gameplay mechanics that can be exploited in the form of a specific combat system, or exploration management, or more or less hard puzzle and mini game to achieve, or all of this.
The concept of Choice is extremely important in a good rpg. Not meaningless choices like you find in 80% of rpg or hrpg :
"What will be my character personality and more important, how the game is going to react to my choices while keeping me interested"
It's useless for exemple to make the player choose between different options, if at the end not both options offer an interesting result. If you get 20h of gameplay and 150 CG by choosing to be a total slut, and only 12h of gameplay and 50 CG is you choose to play your character as a more realistic person, then there is an issue with the game.
Story and immersion are great... but that's not what make a game a rpg. They are what make story driven games good or bad. RPG can have excellent stories too, don't misunderstand me, but before caring about the script in a rpg, the gameplay and character personalisation have to be top notch.