SH 4 is my first sub game.
The reason I liked the look of it was partly because I had had my eye on Silent Hunter 3 for quite a while, and partly because I wanted a game with more complexity than most.
I get a sense of satisfaction out of learning to do new things in a game. It's one of the reasons I like games like Uplink, too - the game starts off very hard, but once you figure out how to perform different actions, it starts to become quite intuitive.
While Gears of War and the like are fun for a bit of mostly mindless blasting and explosions, once you master playing online you just hit the "ceiling" of the game. It's still fun to play, but there is none of that "new stuff" satisfaction.
SH 4 also has the advantage of being a sandbox game, and it's pretty non-linear, which I was also drawn to.
Finally, I'm interested in the history of WWII, which helps I think.
EDIT - Oh, and the difficulty involved in sinking ships drew me in too. In your average game you're just swarmed with dumb enemies who die like flies, which is both silly and rather stale.
A harder target that requires strategy and precision to attack efficiently is a lot more rewarding
