A blunt summary would be that everything is unreliable and disposable. You have to design for failure, because most things will almost certainly fail (or just slow down) at some point.

I learned a lot when I was first moving to the cloud from Adrian Cockroft. He has a ton of material out there from Netflix's move to the cloud. I recommend googling around for his conference talks. (I haven't watched these, but they're probably relevant: http://perfcap.blogspot.com/2012/03/cloud-architecture-tutor...)