Well, the worst aspects of fan films are usually bad cinematography, editing, vfx, acting, and writing. Rogue One has all of those. What makes it like a fan film is the subject matter, to which I would say who cares.
This ^. It pisses me off when people say it’s a fan film. Rogue One is a VERY well made film. It has some of the best CGI (not including Tarkin and Leia) EVER. It’s cinematography is gorgeous and the actors for the most part perform admirably.