Textures and a lighting system that isn't objectively worse lol. There is a reason they had to have special cutscene only lighting. http://www.screenshotcomparison.com/comparison/115387

2014 was a feature complete build of the game by the way and fully playable on PC in February of 2014 from start to finish. You can see this in internal leaks. It had problems on consoles with performance, they was too ambitious and overestimated next generation massively.

https://youtu.be/bX_WePhiYHE?t=2m43s

Particle effects all but gone, water/building/ground tesselation gone, draw distance drastically reduced and replaced by grey fog, and yes objectively better global illumination gone which makes the foliage look a generation older. Witcher 3 retail has the same atmosphere during clear weather as it does when it is cloudy because of the poor lighting.