I like to tackle this one.

Deaths are often used as cheap gimmicks in storytelling. A character is killed often for sheer shock factor alone, but the problem with this is that it isn't meaningful. So what makes a meaningful death? A meaningful death is when a character dies, and that death imparts some kind of meaning. This can be used to move the story, or set a baseline mood, or to start or end a story or arc.

There are a ton of examples of this used in literature, movies, and video games. One example is the death of Obi-Wan Kenobi. This scene is the typical mythology of the death of the mentor, when Obi-Wan's death is symbolic with Luke Skywalker's transition into the hero. He's learned all he could from Obi-Wan, and with his death Luke ultimately learns that the Force is a powerful thing. The meaning here, that Luke transforms from the student learning under the mentor, to the hero.

Transformers Prime uses death to build character as well. I think a lot of people don't see the other point of Breakdown's death other than a MECH plot device. It was used to develop Airachnid as an opportunist who will kill anyone if it furthers her goals.

In video games, one example I can think of immediately is the death of Aeris in Final Fantasy 7. This one is used to move the story and to set up the Chekhov's Gun that will later be used to beat the game and conclude the story. Many deaths in the recent Spec Ops: The Line are meaningful, but in ways I do not wish to discuss due to spoilers.

That's pretty much what I've got about meaningful deaths.