Open world games are this console generation's 3D platformer. Everyone is making them but barely anyone is doing the genre justice. It's the go-to template that many developers will resort to when making cookie cutter game #5372, resulting in many serviceable and mediocre titles belaboured by menial content.
The world could be too big or too small, spread far too thin or incredibly cramped. It could be boring to look at or downright uninspired in scope. Moving through it could be tiresome and riddled with only the blandest objectives to complete - so on and so forth.
Games like Ghost Recon: Wildlands, Agents of Mayhem and Far Cry 4 are far from bad, but remain so painfully average that they stink of being a part of a tired genre.
All of this is why, when an open-world game is done right, it's absolutely outstanding. Games that manage to master rich and detailed worlds are big sellers, constantly being heralded as the best of the best. So, when developers manage to strike gold and make something truly impressive, it's not to be scoffed at.