I've heard people use "trope" to refer to very specific genre conventions, but I suppose "convention" would be the clearest term.
So, what are the main conventions of fantasy? This is actually a complicated question--fantasy novels do exhibit a great deal of variety when examined closely, and FRPs vary from the novels in significant ways. It might be better to limit us to modern, post-Tolkien fantasy. So, what are the conventions? Here's a list of the top of my head:
1. Clear moral orientation. There are clearly identified sides that are good and evil.
2. Grand struggle. The protagonists are generally engaged in activities in which the cosmological balance is at stake (or preparing or training for such a struggle).
3. Fantastical creatures. There are intelligent non-humans that inhabit the world. Specific to the Tolkien tradition would be a derivation from Scandinavian myth: elves and dwarves.
4. Fantastical monsters. The challenges that the protagonists face include incredibly powerful monsters that do not exist, but may have a basis in myth or legend (such as dragons).
5. Magic. There are forces that defy our understanding of physics, and those forces are controllable by some group within the world.
My current campaign, I am afraid, is rather convetional:
1. Moral orientation: there is one, but it is unknown to most of the players, who are distracted by the many shades-of-gray political and theological conflicts instead.
2. Grand struggle: there is one, but it is currently too subtle for the players to notice.
3. Fantastic creatures: In my writing, there are only two intelligent humanoid races: humans and a race of dragon-men. As a concession to players, I have allowed dwarves and halflings, with some modifications. This convention seems to be the one most people modify.
4. Fantastic monsters: Very rare. Through the first four levels, the party has encountered two fantastical creatures (neither from the Monster Manual). Another common one to modify.
5. Magic: I use a homebrew system, but it is a world where magic is uncommon but powerful. Fairly conventional.
Wow. I'm actually disappointed at how conventional my world is. I need to work harder.