as far as characters, original or canon, i can handle either: with the canon people, i want them as realistic as possible. for example, i will not accept picard insulting a fellow officer, even if the guy is such an idiot that makes barcley seem the best officer ever...
with original characters, i'll want them indepth with real personalities and history (how-to-write books recommend that too)
the era/location, etc doesn't matter as long as the style and dialog, etc is good....i read a story on fanfiction the other day, i couldn't finish even the first chapter...it was BAD *shudder*
i want equal parts dialog and action...too much of either gets annoying
i don't care about length, again, as long as it's good. i've (tried to) read 5 page stories that make want to gouge my eyes out and 20 page chapters that intrigued me the whole way
romance doesn't bother me if it's done right