Hi George:
Actually, E-Smith is just an "very server focused" Red Hat. They take a RH distro, install only the critical packages and configure them right out of the box.
At the same time, ES has its own "management" system, based on the manager, templates and so on. This allows for easy customization of the server, and specially for very easy administration (15 minutes to have all rolling).
Plenty RH apps will work on ES, but with some customization. You will need to addapt them to use the template system. In some cases, the needed dependencies will be missing (example X).
In esence, ES is RH tweaked to run a great server, managed through a very simple web manager.
If you need an office server (with some gateway services) its just perfect.
Hope it helps.