You might have heard of a bug from 1.8 that renders fancy leaves on fast graphics, that was supposedly fixed in 1.8.1. Well, here is the truth: They only fixed it with fancy leaves on fast. Vice versa, which is fast leaves on fancy graphics, still is a bug that has even made it into 1.9. Before 1.8, leaves looked more beautiful with the transparency, even when far away. But they look like this now (image on left):

Sorry if this was an intended feature or done for performance increases, but since I have heard nothing about it on the Minecraft Wiki or on other Minecraft sites, I'm just going to say that Mojang did not intend for leaves to look like that. (And don't say that I'm not on smooth lighting, because I tested this on smooth lighting and the leaves still rendered opaque.

Steps to reproduce:

1. Go on Fancy Graphics on Minecraft 1.9.
2. Look into the distance towards a dense area of trees.
3. You will notice that you cannot see through the leaves.

(Affects 1.8, but it will only show an option to say: Affects Version Minecraft 1.9 and future updates.)

Someone told me to turn mipmapping off, and this is what I get (image on right):

For some reason, if I downgrade to an older version, I get this effect by default. I'm not sure how opaque leaves make the game smooth.