Some of the biggest fantasy worldbuilding fails that I see, in no particular order
Gods without religion. The Gods are real and a known historical fact, but virtually nobody is religious.
Cultural racism/discrimination without structural racism/discrimination. Discrimination that exists only in microagressions or mean comments, without existing in any sort of structural way.
Secret history with no clear reason for it to be secret and no clear method for maintaining that secrecy. Major parts of the world's history are kept entirely secret, even though there's not an obvious reason to do so and even when history has shown this is virtually impossible to enforce (especially in a world with any movement or communication across borders).
Large, homogeneous countries. Even without immigration, virtually no country larger than the Vatican will be fully homogeneous in terms of culture, dialect, beliefs, traditions, etc., much less a large one with limited communication technology as is often seen in fantasy. The Planet of Hats problem.
