Q:

What is the difference between "it's up to you" and "it's down to you"?

I see both "It's up to you" and "It's down to you" in conversations.
So what's the difference?

A:

I felt "It's up to you" means that you're the one responsible for the job, "It's down to you" means that nobody else is left who can do the job except you.
ADDING ON: I realized from the comment that what I wanted to imply wasn't completely clear. As ngmiceli says, "down to you" suggests that there were others who could do the job, but for some reason, they are not available to do so (because they may be dead/busy/out to lunch), leaving only you.

A:

'It's up to you' connotes some element of personal choice. 'It's down to you' has more of an implication of responsibility or duty.

A:

Up to you = Responsible for a decision
Down to you = Responsible for an action

Examples

What happens next is up to you
The choice is yours, it's entirely up to you
It's now down to you to impress the judges
Everything that has gone wrong in my life is entirely down to you, you useless...

