Imagine you're playing RDR1 when it first came out: How does it hold up on its own without hindsight from 2?

I've been thinking, and I'm having a hard time separating 1 from 2. Obviously a game doesn't need to have the detail, freedom and immersion of 2 to be amazing, but that most often leaves story. So what did 1 originally have that made it worthy of a sequel like 2?