I've given some thought to it and come to the conclusion that the moniker RPG is not really an apt description to the range of games that it is usually applied to. It may have been once but no longer. In what we usually call an RPG we usually have a character with various statisics that we apply to a variety of tactical situations, usually clothing and weapons are added to adjust these stats and to further complement the characters abilities. The character is also usually tied to a "class" with different strength and weaknesses in the relevant tactical areas of the game. These are the core tenets, deeper story is usually also associated with the genre but in recent years that tendency has declined sharply.

Now here is my main grief, none of these elements actually qualifies as role playing. The standard answer here is that building your character is roleplaying, but These days Its easy to come across games that lets you do quite a bit of avatar customization that would NEVER be classified as an rpg. I therefore suggest that we relabel it into: tactical, and let me explain my reasoning for this. A tactical game is about applying your abilities to overcome a specific hindrance in a strategic enviroment (strategies are the same concept taken to a series of battles) , and the challenges presented in what we usually term as an RPG are always of a much more tactical nature than most other games genres.

Obviously the genre would have an extensive array of subgenres like its cousin; the strategy game. (difference between the two is that the tactical level in a strategy game is always presented as a part of a greater strategic scenario I,E loosing a single battle will usually not give you a game over)

So what to do with the now redundant RPG moniker? I say make it a description of story progression in single player together with Linear and sandbox. where Linear is on rails storytelling (this is your CoD game), RPG is certain extent of control of your characters behavior in-story (this would be your mass effect), and Sandbox a certain extent of control of the surounding world in story(this would be your assasins creed).

Feel free to comment and criticize or improve upon my arguments.