Listen to people describe their idealized cloud, and sometimes you'll hear them say "infinitely elastic".
Not only that, being able to meaningfully grow and shrink infrastructure resources dynamically in response to changing application demand is a very difficult problem -- especially for general purpose use cases.
The application environment has to be able to request and release resources. The supporting infrastructure has to work in concert with the application stack, of course.
And then there's the head-hurting topic of policy definition and management: under what circumstances should appplication "A" get more resources, and application "B", "C" and "D" get less?
And how do you communicate back to the business what real-time tradeoffs are being made, and showing them that everything is compliant?
I don't think anyone has complete answers for most use cases, but some interesting work in this area is starting to manifest itself.
In particular, the VCE crew is presenting their current progress on a solution dubbed "Application Lifecycle Manager", or ALP for short. And it's an interesting discussion, to be sure.