I've been keeping track of the updates that will become Minecraft 1.7, and they look promising. Not only does it appear that a huge number of new biomes and "rare" biome types extending existing biomes will be added, but there's something a little more to the story...
I recently decided to try the latest snapshot, and Minecraft feels completely different. The frame rate is quite smooth, it's hardly choppy, and the new mipmapping feature improves the appearance of forests by smoothly interleaving the textures at range. I'm not sure precisely why this is happening, but I suspect they've squeezed in support for hardware that implements newer versions of the OpenGL spec. I'm not sure what this means for older hardware or whether this may also be due to other improvements made to the game, but it's possible that those of you who have had issues with reduced frame rates since prior to Minecraft 1.5 may note some improvement once this version is released. If you're particularly brave, you can use the new launcher to try the latest snapshot--but be certain to heed the warnings and configure it to use a different directory!
This is gonna be awesome!
Below is a screenshot of a taiga. Note the improved appearance of the leaf textures in the distance. Also, some new visual effects have been added, such as a darkening of the screen when standing in a shadow in full daylight.