The Mummy was the first fandom I was heavily into. I discovered fanfiction for it about seven years ago and read a lot of it before beginning to write my own. The main reason I like The Mummy is very similar to the main reason I like Lois and Clark, the characters are wonderful. Rick and Evelyn have such amazing chemistry, as do Lois and Clark. I have to admit it is mostly the fluffy romances I usually go for in both fandoms. As for what The Mummy has that Lois and Clark doesn't? The only thing that comes to mind is Brendan Fraser... but Dean makes a very, very good substitute.
I've attempted to get into other fandoms but never quite caught on.
Moulin Rouge is my favorite movie other than The Mummy, and I've read a few fics, but I just couldn't get into it. Maybe the movie just seemed to finalize everything too much?
I've also attempted Silent Hill, because the psychological aspect to it is absolutely amazing, but fanfictions just don't do the hauntingly beautiful art (and yes, these video games are ART) justice the way that playing the games or watching the movie does.
All in all I really think that the biggest factor for me is the magnetism of the characters, and Lois and Clark is LOADED with it.