#1 - Get the physics right. Most clothes seem to be made of rubber, not cloth - they are way too stretchable. And take a look at the surface friction of the stuff you put in game. Blankets usually don't slide off a bed as soon as they are moved a few centimeters/inches. And if that Big Black Dildo is rammed into an asshole, it will not just easily slide out again. Movement of the girls and the player's avatar needs to be faster and more fluid.
#2 - Get the girls right. They need way more brain. The parser has to be looked at. And the girls need some personal traits - take a look at Artificial Girl 3 by Illusion for that; they got a (basic) system where traits are achieved during gameplay. The girls should act and react better to what goes on around them. So a girl ight start up with an "angry" mood, but after being put into the stocks and whipped/caned/fucked for a while, she might turn to "frightened" rather.
#3 - More - sex - positions. And more actions the girls can do, like spreading their legs when ordered to, even when standing. Putting their arms on their backs, to cuff them etc. etc.
#3 - Please get rid of the oversized rubber-cocks. People with a gigantic dick fetish can trust that there will be modders satisfying their needs. But those arm-long and arm-sized dicks just make problems when it comes to interacting with the girls - Mummy Face Blowjob, anyone?
#4 - Please get rid of the fuckwalking. Let the feet of the player avatar get stuck to the ground on the press of a key and then the keys used for movement move the hip only... shouldn't be that difficult to implement - the girls are stuck to the ground most of the time when fucking...
#5 - some items - the fuckmachine for example - should be moveable. And I mean EASILY moveable, not like the benches, which can be moved by running into them, but are impossible to stand up again, when they fall on their side.
#6 - SPEECH!!! And not done by some lousy porn actresses - like it's done in other games *coughsexvillacough* but more like in Artificial Girl3 - done by professional voice actrices...
That's just a few things that came to my mind... there sure are lots more, some of them are already mentioned in this thread.
edit: inserted #6