What do you look for in a piece of fanfiction, as a reader?
Good characterization is possibly the biggest thing for me. I look for whether the writing matches the voices of the characters; their manner of speaking, how they interact with others, how they would react in certain situations, etc. There are some instances where I can accept a little bit of out of character writing, though nothing too extreme that is beyond believability. Having a good premise for a plot and then delivering on that is another thing I look for. Sometimes I've come across something that sounds good, but the outcome isn't what I was hoping it'd be and it leaves me feeling disappointed. It's even worse when it starts out fantastically and then ends on a blah note. So, writing consistency is something I tend to watch out for.
I tend to read mostly oneshots. Long, multi-chaptered fics I save for when I know I have time to read through it all, and even then I only will read those if they're completed. I don't have the patience for WIPs. When it comes to an OTP I will read almost anything, within reason. But I mostly love reading about them getting together, whether it's them being romantic dumbs or oblivious idiots. I enjoy my fluff just as much as my smut. Angst is only reserved for when I am emotionally prepared for it, and I actively avoid character death fics at all costs.
Simply put, what I look for in fanfiction is similar to what I look for in anything I read. It doesn't necessarily have to be perfectly or extraordinarily written by any means, but it has to be well-written enough to get me interested, to keep me engaged and invested with the story and the characters and their relationships.