The scale of technical achievement is by far more impressive than anything that came before. Metal Gear Solid 5 might be the closest thing in terms of open world technology, but RDR2 is far superior to it.
The anti-game aspects create a very different dramatic experience than most games. Sometimes you just ride along with another character and do nothing but observe what happens. Other times your plans get derailed in an Inception sort of way and you end up halfway acrosd the map doing something you never intended, none of which has anything to do with the story. This happens in ways that are much more organic and natural than previous similar mechanisms in e.g. GTA games.
The story deals with a lot of themes that have more impact because of coming across them in an open world (stumbling into KKK meetings, seeing lynched bodies hanging in swamplands, tracking down a gang that tortures animals, finding evidence of families split by slave trade, and many more).
The main story is compelling and well crafted, but the fuller picture from the pastiche of stuff in the world makes it feel effective, and the scenes of blood-thirsty wild west action feel more like uncommon punctuation marks than important aspects of the plot.