What makes Gothic 2 shine (apart from the high difficulty) is the setting, it somehow feels more "alive" than it did in Risen. Not saying that Risen's setting was not good, I thought it was, but Gothic 2 had a better setting (even though the two are relatively similar). I would still recommend playing Gothic 1 first though (in the gothic series, that is), because not only does the events in Gothic 1 play a large part in Gothic 2, but the game is, as I said earlier, more forgiving and easier to get into. Also, even though Gothic 1 might not be a looker, it had a really interesting, and quite different, setting. And this is from a person who has no nostalgic ties to Gothic, I played them relatively recently (I bought "Gothic universe", a collection of the first 3 gothic games).
Oh, and the expansion for Gothic 2, Night of the raven, does apparently increase the difficulty of the game by quite a lot. The expansion is integrated into the main game though, so there is no "play the game first and then the expansion". My version did not allow me to play the game without the expansion.