Minecraft wiki entry for Development_versions, associated text: New mineshaft type that generates in mesa biomes (including all variants).
Placed so that the midpoint of the structure is near sea level, rather than so that the top is 10 blocks or more below sea level.
Uses dark oak wood for wooden structures.
New NBT tag MST on mineshaft structure pieces to indicate the type. 0 is the traditional mineshaft, 1 is the new mesa mineshaft.
Features generate differently if above ground (i.e. the sky light where they generate is 8 or more). This is not dependent on mineshaft type.
Cave spider spawners no longer generate if sky light is 8 or more.
Various wooden pieces no longer generate if the block above is air and/or if sky light is 8 or more.
Rails in corridors are more complete if sky light is below 8.
90%, versus 70% in 1.9 and in above-ground corridors.