Okay, first of all, "Open World Games" is not a genre. It's not even a "mechanic" or a "setting".
Rather, it's an element of level design. It's a base part of gameplay building that isn't inherently good or bad - that all depends on how it's used in the greater context of the game.
The two big open-world franchises, TES/Fallout and GTA, use open-world design for drastically different reasons.
In GTA games, the core gameplay that pulls people back in again and again is the run-and-gun, frenetic, dynamic, highly randomized chase/shoot out sequences you can trigger by accidentally backing over a cop as you pull out of the mall. These episodes are fun because you never know what's going to happen next. It's immersive because you react (to a certain extent) like a real fugitive would. You can go left, right, ahead, back, make a stand, highjack a helicopter, or scram for cover (paint job). The decisions are realistic, and that's where the game generates the bulk of its entertainment value. The open world is necessary to facilitate this sort of gameplay because you can't have all of this in a corridor.
In the TES/Fallout games, the open world exists because the core gameplay in those titles revolves around exploration. The strength of these series is in allowing the player to ask and answer, "What's on the other side of the hill?" It's about going on long hikes, getting lost in the wilderness, seeing the sun crest over a distant mountain. For all of these purposes, you need an open world. You can't coax that sort of exploration-hunger out of a linear path. A linear Bioware game can't tell me I'm "exploring" a new alien/fantastical world when they're railroading me from one set battle sequence to another.
The point I'm getting at is that GTA and TES have very, very dissimilar core gameplay, but at the same time they both use the open world feature well.
So, if nextgen games want to succeed in using open world games, they have to start from the ground up and ask "What is the core gameplay going to be and do we need an open world to make it work?" Simply because your setting is cool and you want to show the player more of it doesn't cut it.
For the Witcher 3, I can see it working... although tbh I think the first-person camera is the other half of the formula in TES/Fallout games and as CDProjektRed doesn't have that, they're going for something else that isn't exploration. What exactly, I'm not sure.
For Mirror's Edge 2, it makes no bloody sense at all. The game is at heart a racing game. You run the same course over and over again until you nail it down to perfection. Open world doesn't add anything to that experience. Sure, Faith's world is really interesting and I'd love to see more of it, but that should be in a different series with totally different core gameplay mechanics.
Two side notes:
1.) Far Cry 3 is an example of where the open-world level design and the gameplay fell apart a bit. It tried to do the frenetic randomized run-and-gun of GTA, but wasn't as good (for a variety of reasons). At the same time, the core CTF gameplay didn't rely on the open world. And because you move linearly from one end of the map to the other, there's no real added benefit. Integration fail. But, yeah, it was hella pretty.
2.) I tend to think of Paradox Development Studio games as open world (Europa Universalis, Victoria, Crusader Kings). You start off in a world with its own rules and 'engine' running the AI. The game can 'play' itself without you. You're dropped in as a character (country or ruler), and can muck about to your heart's content. I guess the same could be said for Civ games, but those have a number of predetermined "win" conditions...