Environment: Environment Detail
There are five options for Environment Detail. According to the tooltip, this setting "controls how far you can see objects. Decrease to improve performance."
Don't confuse this for the previous setting called View Distance, which controls how far ahead you can see, period.
Set to low, you can still see quite a ways in front of your toon. However, there's a lot of detail missing that you get with the Fair setting below, including the waterfalls directly ahead and trees off to the left.
Turned up to Good, even more detail comes into focus. Trees that were previously missing now obscure the structure to the left. There are some floating elements of the environment that appear on that side of the screen, too.
More of the left side of the screen is visible here, but there's frankly little improvement in this particular screen shot from turning on High Environment Detail.
Trees that were previously too far away to see are now visible at the Ultra level. Depending on whether or not you play competitively, being able to see far away (and see the objects that far away) may be more important than some of the detail-oriented settings. I put Environment Detail pretty high on my priority list of settings to maximize when possible.