Author has written 16 stories for X-Men, X-Men: The Movie, StarTrek: Other, X-Com, TV X-overs, Mechwarrior/Battletech, Robotech/Macross, X-overs, Marvel, New Jedi Order, and Star Wars.
I used to write a lot of fiction, though considerably less in recent times. I pick the fandom depending on what kind of feel I want the story to have:
Buffy the Vampire Slayer and Highlander the Series lend well to stories in a modern setting with elements of the fantastic.
Star Trek shifts the focus to discovery, something that's missing in most settings.
Star Wars allows for the exploration of ideological conflicts (what is "Good"?).
Marvel Comics introduces fantastic powers as an analog for the emotions and events we deal with as we mature.
DC Comics deals with conciling role-models and real life. It's hard to do when the lives of the DC characters are determined by internal politics as much as what the writers really want (kind of like real life, in that regard), which is why fanfiction is such a great refuge for those characters.
The most enjoyable stories to read are ones that introduce new possibilities, are true to the characters and setting, have good readability, and a writing style that suits the story and author. A coherent/semi-original plot is also a plus, but you can't expect everything.
The single most important considerations in critiquing a story are: Did it fulfill its objective? If so, how? In what ways did it fail? How can the objectives be better met? What elements in the story were unnecessary? Large complications that serve no purpose, or having the story expand beyond its scope usually fall into "unnecessary elements."