Recently on a fresh install of SharePoint 2010 the following were observed:
Searching would give an error
Going to Search Scopes in the Site Settings would create the following error:
HTTP Error 503, The Service is Unavailable
Going to Term Store Management in Site Settings gave the following error:
The Managed Metadata Service or Connection is currently not available. The Application Pool or Managed Metadata Web Service may not have been started. Please Contact your Administrator.
Checking "Manage service applications" and "Manage services on server" both showed the Managed Metadata and Search services to be started.
Looking at the server in the Internet Information Services (IIS) Manager and clicking on the Application Pools it turned out that the "SecurityTokenServiceApplicationPool" was missing. For some unknown reason this did not install.
A wipe and fresh install was required to fix this.
Note that if the application pool does exist, make sure it is Started