Minecraft wiki entry for Development, associated text: General
Updated tree generation to not make giant green matchsticks
New logs and leaves textures. It is unconfirmed if the gravel and sand textures were changed in this version, but it is likely they were not, since the terrain.png file found in 0.0.15a Multiplayer Test 1 dates to May 31st, 2009, three days after this version was made. This, coupled with the fact Notch never said anything about those textures, may mean they were changed at a later date.
Notch experimented with dark fog in this version. Fog would change color and become darker depending on what blocks the player was looking at. Later removed due to buggy behavior.