Generally, you'll see the terms 'good' and 'great' thrown around when discussing video games. In your opinion, what separates a good game from a great game?

The same different between RE4 and RE5.

Other than semantics most of the time, I'd argue that a great game is one that provides you with memorable, remarkable experience that lingers or gets stuck with you even after you have finished it. A good game gives you enjoyement, but don't have to be particulary distinguished or catchy. 

One has boobs the other one doesn't

most solid argument i've ever heard

On a serious note, I would say it comes down to the little details. Whether it's realistic movements of water and shadows, attention to detail in environments, small decisions having visible impacts, etc. The Witcher 3 is a perfect example of a great game where the little details were stressed. It really shows the developers were truly passionate about the project.

Exclusivity. A good game cannot be great if it is not bound to a platform.

Tlou and uc2
One is a good game and the other is a great game

One entertains you and the other doesn't.