I think everybody gets what a "trope" is by this point, but what about things that go beyond even that? There are common things that might show up in TV shows and books, and then there are things that are apparently required to be central to the plot.
As an example: In an urban fantasy that's narrated in the first person by a female character, it is a requirement that two or more hunky supernatural creatures will lust for her. This has apparently spilled into science-fiction narrated in the first person by female characters as well (hi, Hunger Games!), but as a fan of strong female protagonists and urban fantasy I repeatedly run into this exact same scenario over and over again.
Anita Blake takes it to a scary level (I think her lovers are in the dozens in the current books, but I stopped reading them), while Twilight and Merry Thompson stick closer to having an alpha couple with one forlorn guy on the outside.