A massive amount of content
Some open world games are massive in sheer size, yet they offer little to nothing for the player to do in some parts. While GTA 5 isn't perfect in this regard, there is still an astonishingly high number of activities to do.
For example, Trevor Philips has several exclusive features. Hunting, bail bonds, and delivering people to the Altruist Cult are good examples. Each gameplay mechanic plays differently from one another, and they may have the player be in a different part of the world from Los Santos.
Like other notable open world games, GTA 5's world is big. Hence, there is a good amount of both quality and quantity to consider when it comes to content.