There are three options for the Sunshafts setting, which was only recently added to the game via patch 4.0.1.
- Disabled: Traditional sunshafts and glare
- Low: Depth-based sunshafts and glare
- High: Depth-based sunshafts and glare with improved sampling to reduce anti-aliasing
I'm guessing that Blizzard actually meant "...to reduce aliasing" on that last option.
With that said, it's actually pretty difficult to tell the difference between High and Low. What matters is that you play with sunshafts enabled. It's really very well-implemented, and it's an attractive effect if you're the sort to hang around Theramore and watch the sun set.
If you want to see this effect in action, you need to be in a zone where the sun is actually out (and not obscured by clouds, as it is in many places). Look through trees, or simply blot out the sun with your flying mount. Anyone who happens to be below you can fight in the shade.