Yes, the visuals are a large part of that. Next to games like Fallout 3, Enslaved was a truly lush and diverse take on the post apocalypse with far more appealing visual characteristics and touches. We can probably thank the heavily linear design for that.
But it's worth noting that the soundtrack was also rather great. Without exaggeration, it was largely atmospheric and a big part of the experience in its own way. Take for example, the piece posted this fine morning - perfect for waking from a dream with thoughts of strange and empty places fading in and out of view.
Or just relaxing to over a cup of coffee. That works, too.