Wikipedia opens up to describe an operating system as "a collection of software that manages computer hardware resources and provides common services for computer programs".

Since the majority of the programs we build today run on servers, not desktops, we needed new APIs to compute / storage / network. A new class of OS provides these, which we call IaaS or Cloud Computing. Some of these new OSes also provide the aforementioned 'common services' such as datastores or event loops.

I think the excitement lies around having an open source OS for programming your infrastructure to, instead of a (nevertheless excellent) proprietary one.