If you want to check out the differences between Gears of War: Ultimate Edition on Xbox One, and the original game on Xbox 360, Microsoft provided us with some really interesting screenshots.
Each of the pairs of screenshots below shows first the Xbox One version, and then the original game on Xbox 360.
Xbox One
Xbox 360
Xbox One
Xbox 360
Xbox One
Xbox 360
Xbox One
Xbox 360
Xbox One
Xbox 360
Xbox One
Xbox 360
Xbox One
Xbox 360
Xbox One
Xbox 360
Xbox One
Xbox 360
Xbox One
Xbox 360
As you can see, there has been a massive evolution of the graphics. Lighting has been improved radically, object geometry has been revamped, and there are even new models all over the place in order to make the environments more dense.
The Coalition definitely went above and beyond the call of duty to turn the game in one of the most extensive remasters (you could easily call it a remake, and not be found wanting for it) in the past few years.