A lot of people have been adding the Assassin's Creed series into the mix so I'll address all three games here.
In open world games, I feel what distinguishes one game over another isn't the mechanics or even the overall concept. Yes, it is damn impressive to create a near perfect representation of 15th century Italy, and the fact that I can say I've explored it is a pleasure of mine. But, let's be honesty, why am I running around that open world of Rome or driving around in a funny parody of New York city? The great flow of the free running mechanic in Assassin's Creed and the ability to drive over pedestrians while listening to those funny radio shows. While those are great things, let's face it: those have nothing to do with the finely crafted and detailed locations and environments within the games.
I have to be honest that when I think back to playing Assassin's Creed 2, the only thing I can remember about the open world I just explored is a blur of Florentine buildings and streets with carts of hay at every street corner. I know there are unique locations, but in the end what makes the open world aspect of the game memorable is the mechanic of running, jumping, and climbing, not the world itself. Why is that? Well, probably because I'm not actually exploring the world. I'm getting from point A to point B as quickly as possible. Same with GTA. Same with most open world games these days (I can't talk about Red Dead Repemption since I haven't played it). In games like the Elder Scrolls and Fallout, the entire open world mechanic is the open world, i.e. exploration. You want to find new locations, talk to people you find on the road, look for new and interesting places. Finding these new places and people will introduce new experiences, quests, and stories. They aren't all laid out on your map; you have to look for them. Any location you find, you add to your map and now have the ability to fast travel back to it. Nothing is just given to you, you need the desire to work for them before you can have it. Bethesda does this like a pro, not only having memorable locations and characters, but giving you the wander lust to explore and seek them out.