I only read fanfiction starring my favorite canon characters. Yep, I'm picky, but I've never loved Star Trek for the world-building. I love it for the characters.
The other criteria is that it must be well-written. Professional quality, or at least close to it. Writing is my job and I can't ignore massive mistakes. They make my head hurt. It also has to capture my favorite characters accurately.
The other things you listed don't really matter to me. Long or short, action-packed or romance-focused, it's all fine with me.
Yep. That's exactly what I was going to say.
I'm not a huge
fanfic reader, so when I do read it, it's because I want to see a logical continuation of an idea not fully explored in canon. Obviously I want it to be something I could easily see as fitting into canon, because if I wanted to read an original fic I'd just read...an original fic. And it's the characters I care about, so the more OCs a fic has, the less I'm interested, usually.
And yes, professional-level writing is a must, haha.