Been a long while since I've played Melty's, but for the most part I kind of just recall it being, albeit rather high quality and pretty expansive, mostly standard faire RPG Maker stuff. Karryn's Prison isn't a perfect game for sure, but the gameplay systems themselves do a *lot* for me in terms of enjoying it, both as far as the hentai goes, and it being an actual game.
Of course, could definitely see how someone might have gripes with the gameplay itself after playing for a bit, but its a definitive classic as far I'm concerned, and it's at least caught on somewhat considering there's now at least 2-3 games emulating a similar type of combat sex/corruption and stat tracking and all that. Really the only thing I DO wish it had more in terms of fetish content was some form of pregnancy/breeding, but hey, at least mods have pretty much covered that.