Leaves rendering on fast graphics has been changed since the 1.8 snapshots (14w25b), and their textures no longer blend correctly like they did previously. Instead, pixels that would've shown otherwise transparent on fancy graphics render as pitch black on fast as can be seen here:

This was caused by the removal of the opaque leaves textures and has been present in the game since then. See comments for more info.

Original description

BEFORE READING/JUGDING NOTE THAT MOST PROBLEMS OCCUR WHEN USING A RESOURCE PACK BECAUSE THE ABILITY TO MAKE OPAQUE LEAF .PNGS WAS REMOVED.
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The only problem with the default resource pack is that the dark parts of the blocks (in between the "leaves") are much darker than the previous versions.

When using a resource pack however, the leaves seem to do that odd "gravel" thing that you fixed for the default pack in this version.

I also see you removed all the opaque textures from the .jar expecting that the game will do a perfect job filling in the clear parts from the fancy leaves, but it doesn't.

If you look at the pictures below, you'll see that the jungle leaves have the most obvious difference.
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Possible Solutions
-Add opaque .pngs back for fast leaves
-Or you could make a way to choose what color you want to fill the cracks with.
-Or you could also make something like "leaves_birch_overlay" for the fast graphics