It really all depends... but that goes for any genre.
I'm pretty open minded, so I like almost ALL genres. So, what makes a game fun for me, goes WAY deeper than it's genre.
And simulation isn't any exception.
Now, I'm not too fond of First person shooters- but that doesn't mean I don't find any FPS fun. I love Metroid Prime, I like some 007 games, I liked what I played of The Conduit, I like Halo a little bit- but not for the reasons most other people do- (Heck, I'll just tell you right now that the only reason I like Halo was for the editor mode in Halo 3), and I like some others. The reason I have a dislike for First Person Shooters in general, is the fact that most of them are WAR/ARMY based ones, and I really don't like that. I think it's ugly, stupid, and sometimes- I even feel that games like that, (the ones that are just- realistic war fields) don't even deserve to be called a video game. I feel that at the core of video games, there should be some element of fantasy. -like in The Conduit, there are aliens. But I digress... My point is, is that it's not the Genre alone that makes a game fun. There's more to it.