Also, forgive me if I misinterpret how some of the code works - I've only just started reading the code.Anyway, on to the main event:
I noticed that as a chunk is generated it creates trees and grows grass on the mud. celeron55 may already be aware, but as the chunks are generated the blocks at the top of the chunk do not seem to account for what will be generated (or not generated) above them. For example, mud blocks at the top of a chunk (call it chunk1) that receive light will be converted into grass. However, when a chunk is created on top of chunk1 there may be more mud/grass blocks placed on top of the grass blocks in chunk1. This problem can create grass blocks that are 'buried' under other blocks, and I'm assuming that was not the intent.
Similarly, trees in chunk1 that would have leaves extending into the chunk above it end up looking like they hit an invisible ceiling once the chunk above chunk1 is generated. Even though the blocks above the trees/leaves are air, the trees/leaves stop 'growing' once they hit the vertical chunk barrier.