I’d definitely say that there is a massive emphasis on romance - often this romance is actually pretty toxic or unhealthy, but portrayed in a romantic, glorified way.
Other than that, I’ve seen stories covering all sorts of themes getting into the top 50. I was in it for fantasy for a while with a completely different story idea - until I didn’t update for a few months. I think that a lot of the people in the top 50 know what appeals in terms of a story, a cover and a synopsis and use this to their advantage. They seem dedicated to updating frequently and giving the reader enough information to be satisfied.