I can’t possibly count how many video game characters I’ve played (and story protagonists I’ve read about) that had amnesia of some form or another. As Azuriel pointedly puts it “Amnesia is so cliche a plot element that merely saying it is cliche is itself a cliche.” It’s an easy way for game designers to explain why a character needs to be explained all that happens around her and why she needs to ask the most basic of questions. I’d say that there are other (better!) ways around the issue though.
The best way, I think, is to do it just like many authors do it. Throw the player right into the story and explain nothing at first. Those things that really have to be explained can be done stealthily. Have the player character pass by a classroom in which the teacher tells her class about the geography of the world or have them stumble upon a book that explains background information on the current king. What’s even better is to allow the player to infer information about the world from conversations and actions they witness. Have them build their own picture of the world through observation, not exposition.
There’s no need to tell the player of Dragon Age: Origins about the Xenophobia of humans and they way that they treat other races. Instead, you simply get to witness behavior and conversations that let you draw your own clues on how the world works. Maybe it doesn’t have to be done in such a heavy handed way as DA:O does, but the concept is still sound.
Dragon Age has another useful concept for explaining further details about the world and pretty much anything that goes on in the game: the codex. The codex is a book that automagically fills itself with information on pretty much everything you encounter. If you don’t care what a revenant is, you can just kill it and move on. If you do, you can check out the codex entry that conveniently unlocked the second that you met your first revenant.
Some might argue that this is immersion breaking in nature because it makes no sense. (Why would my book suddenly fill with information on things I knew nothing about before?) I’m not sure if this is actually a big issue (suspension of disbelief and all) but even if it is, what’s there to say against a codex that is already filled with information from the start of the game? As long as books have been invented in your game world, there really isn’t any reason against some sort of compendium full of information that a citizen of your world should know. You could even use it to enhance immersion by including inaccurate information that you could then (automatically or manually) annotate as you find out things that are not common knowledge. And wouldn’t that make for a nice inclusion in your collector’s edition? Or swag to sell?
I’m sure there are other ways out there that help you avoid amnesia (or similar excuses) without having to drastically alter your story or include a lengthy exposition arc. (I’m looking at you, Fallout 3.) Whatever you do, please keep away from amnesia. I have actually put away games within the first five minutes of playing because of that lame excuse. Make a bit of an effort, it isn’t all that expensive.