To me it would be that Western games are mostly in their adolescence and in that adolescence also in an age where anyone can make one. As a result Western games are often very similar, not much experimentation has been done yet. Japanese games have been developed longer. The line that Japan in particular tends to stride between sexual expression and repression is confusing. Much of the repression in their culture comes from them adopting western values. There is seemingly a war going on, with society as a whole appearing to adopt the repressive views of the west, but with people in private seeking release. As a result, their porn is also a bit more depraved, involving many different forms of rape fantasy, from train groping, to actual rape, to freeuse, etc etc.
I personally enjoy both, but I like most of all when the west adapts the VN style of Japanese games. Too many western games are boring life sims where you have to do a series of mundane tasks every day and on top of that play games to try to climb steps with a girl. VN's offer a more coherent story and allow progression to be less linear and more organic, in my opinion.
