We can choose what something means. Choose if it is good or bad, right or wrong. Choose if it's worth spending time on and so on. That's the biggest difference. We are more than our instincts.
I think Trysta hit it well with language. <br />
I think it's because man is the only creature worried about his rank in the animal kingdom.
I would say that the biggest difference between animals and humans is that we can convey thoughts remotely--as in not needing to personally be there--via writing and drawing, and animals can't. They can convey physical information by scent but not thoughts or emotions.
There is no fundamental difference between all creatures. There are differences in their mental and physical capabilities.
Interesting answer, Promethius50, but I don't believe it. I think that most people would agree that imagination, hopes and dreams are all related. I watch my pets dream all the time. I watch my dogs hope that I will drop the food I'm preparing or eating so they can get it. Just because they can't speak and don't have the manual dexterity to write or draw doesn't automatically mean they can't imagine.