A potentially large optimization that Minecraft is lacking is "greedy meshes". Essentially if two quads are next to each other on the same plane and with the same texture you can merge the quads and use fewer triangles. One way to do this in the shaders is to use textureGrad for the texture repeats.
There are definitely some downsides to this:
More complex mesh building
Slightly more complex shaders
More data needs to be passed to the GPU
This is a feature request, not a bug.