The bug:
Various blocks are lit inconsistently with varying levels of darkness. This results in strange looking builds and structures. I noticed that obsidian seemed a bit too dark, so I replaced a few textures with a completely flat gray 16x16 texture to compare.

How to reproduce:
Place down blocks like obsidian, sand, and any slabs or stairs, and compare the way they are lit. The differences are much easier to see with this resource pack. I didn't get a chance to replace every texture so there may be more inconsistencies than this.