Lots of new and interesting things happen when you move to the cloud that you didn’t expect to have to deal with before (e.g. how to automatically bootstrap auto-scaled instances, etc.) One area that has a lot of complexity and uncertainty around it is certainly cloud security. As someone found out on OpenStack, there are things you rarely ever needed to think about, like the RNG not being random enough and doing things like generating the same SSH key multiple times. Not that this couldn’t have happened outside of the cloud, but as you scale systems and spin up instances dozens or hundreds of times a day, problems with small chances of occurring can suddenly start to appear at an alarming rate.