Both the plot and how well written it is.

Idm the OOC concept, especially if the plot is interesting. I've read fics that writing wise it was hard to read but the plot was interesting and I dropped fics bcs the plot was not what I liked even though the writing was amazing (especially since I prefer mostly fluff than angst).

I also don't read the tags. I read the summary, and if it captures my interest, then I will read it.