If I'm not remembering it wrongly, he is simply more cynical and, in general, disinterested in most things around him other than sex and staying alive, as long as his actions abide by the code of conduct that he has set aside for himself

I don't know much about the other witchers, including Triss, since I don't recall any of them being mentioned in The Last Wish.

Basically, witchers should be without emotions, but something didn't quite work and they do have them. However, Geralt is ... Let's say very vulnerable trough his emotions, so he put this shell of acting like a "true witcher" around him. Other Witchers from his school are older and far more natural.

Agreed that the game takes a bit to open up, I didn't like the combat at first either but I got to like it more after some time with it. If you're on the fence I'd wait and see if it goes on sale over the holidays which I think is quite likely. I bought the game for $10 originally and paid $5 to have it again when it was released on GOG and am quite happy with my purchase.