Deforestation and Birds

It would bring more awareness to environmental issues and make the game more interesting if cutting down a whole forest impacted the game in some way.

I think stopping most animals from naturally spawning in those areas would be the best way to do it. If the game had birds (and if parrots were seen soaring in the sky), you would immediately see the impact if suddenly the skies had no birds after cutting all the trees.

You would still be able to clear areas and make bigger builds without the grass color changing or something. That'd be a bit much for vanilla Minecraft. And you would still be able to replant trees to get the animals to return.