I've seen some stuff about this. Here's what I remember:

-Domesticated animals, especially certain breeds of dogs, may have a hard time fending for themselves at first. Probably starving to death or something.

-Most man-made structures require upkeep. Things would fail or shut down. After a few years, buildings and bridges would collapse. Not sure about stuff like nuclear reactors though, I'd assume there's some kind of shutdown if no one's around.

-After a few centuries or so, everything would have collapsed, become buried, and covered in vegetation, making it appear as if we were never here.

-Life marches on without us.