Sure, these days have almost every game sporting the newfangled 3D, but way back when, everyone had to live with plain old 2D. 2D, or two dimensions, limit the game to scrolling backgrounds, but some games even now make use of this basic concept.
A staple in modern shooters. They hide, crawl, creep, climb and jump, and are usually pretty weak but hard to hit.
Extraterrestrials appear in many video games in a variety of styles. Sometimes they're evil, sometimes they're benevolent, and sometimes they're very similar to humans.
Alternate realities are worlds that exists next to, in parallel of, or in place of our own.
An often used plot device where the main character wakes up and has lost all their memories. The game usually centers around getting your memory back or uncovering the events that lead to your confusion.
A dark power that is usually the cause for the world's strife. The curse can only be broken after the protagonist (or band of protagonists) defeats several dangerous trials.
Artificial Intelligence, or AI, commonly refers to the programmed behaviors of NPCs in a game, whether it's the fact that a soldier takes cover behind a wall in Gears of War, or that townspeople cheer upon your arrival to their village in Fable.
Urban legends, particularly horrific ones, have come back into vogue among Internet communities, where they are referred to as "creepypasta". Some of the more popular examples have been adapted into video games.