As Sameer mentioned earlier that in cloud IaaS you get access to shared resources, but for CPU, HDD-space, RAM you get a guarantee. However it is also important to note that AWS (& other IaaS providers, I'd imagine) provide different options where the no. of processors, amount of RAM, and I/O performance can be light, moderate or extensive. Apparently some providers allow you to mix-n-match cloud-hosting and colo.
It is a commonly held belief that in cloud service s.a. AWS, to make the best use of their infrastructure, you may require a completely re-architected/redesigned system, which leverages the strengths of cloud infrastructure. Take the google live-search feature for instance, or Youtube for that matter. I am sure you'd agree that these are pretty i/o intensive usage of cloud infrastructure. Of course, we may not have he luxury of doing so, always or immediately.