What I don't like about Fedora is its release cycle, forcing you to upgrade every year or so. With Ubuntu I can still be getting updates for a sytem installed four years ago now. I also don't like the lack of many packages inside the official repositories, compared to Canonical's. And lastly, well, I feel a little bit uncomfortable when using their specific tools like, for example, dnf, or their configuration conventions, which differ from that of Debian based distributions. This is a matter of taste, though.