When I started off on the net, I dabbled in a few fanfic fandoms, as I was strolling around USENET. X-Files, Highlander, Xena caught my eye for a moment, and Sliders for a longer time. I found Sliders about the same time as L&C, and was fairly stuck in both. Sliders dwindled off after the third season (stoopid Kari Woowoo. Stoopid David Peckinpah), and for the longest time, I kept sights on L&C.
A few years ago, I stumbled across some Stargate SG1 fic, and I've been fairly stuck there recently. Themes are usually team-based, and some romance-based, but only with certain ships. I guess like most people, if the ship doesn't fit into my perspective, I'm not interested. If the morals don't fit mine, or what I think the characters would do, I'm not interested. Incongito mentioned cheating being acceptable in some fandoms, but since it's not acceptable to me in any fashion, I don't want to read about my favorite characters doing it, even if it's with each other.
I've dabbled in a few other fandoms recently, when something catches my eye. I've had problems getting into Doctor Who fanfic for the same reasons Sara mentioned. Harry Potter, it depends on the setting.. I like post-school stories, but once again, only with the ships that fit my ideas.
What attracts me to the other fandoms is the new ideas. After 10 years reading almost solely L&C, I'm getting burned out. I like the different interpersonal relationships (intimite or no). I like the different personalities. And I like the different ideas. It's like being stuck in the same town year after year after year, and finally moving away. There's just so much new stuff to explore!