I've been playing for around two months now and I'm still not sure about how this works. Is there any time when it's acceptable to roll greed on drops? I've been mainly rolling need if I really do need it or pass if I don't, but now that I'm lvl 60 on my first class, I can't help but to think that some of the stuff would be helpful on my next classes.

Also, I've seen the "<insert item name here> could not be obtained" message, so my deduction is if everyone rolls pass, the loot is lost.

So how does it work exactly?