What impact would there be if humans disappeared from the Earth?

So what would happen? Animals would break free and start Animal Farms, of course.
The oceans, lakes and rivers would self-clean.
Roads and cities would become forests and jungles.

But what else would happen?

And what the heck happened to all the humans?
Plague?
Nuclear holocaust {bombs, power plant explosions [Japan 2011]}
Zombies?

{I got this idea direct from a reply I just read by Cruiser}?