The single most important function on a Smart TV today, in my opinion, is DLNA support.
DLNA is an acronym for Digital Living Network Alliance (DLNA), a set of guidelines that allows the sharing of digital media between devices that support DLNA.
Many receivers support DLNA today including low-cost ones from Pioneer. DLNA is not just restricted to Smart TVs.
An example of a typical usage of DLNA would be if you have a DLNA server setup on your PC/MAC computer and a DLNA enabled Smart TV on your network, you can browse and play all your audio/video content on your Smart TV.
A DLNA Server can be setup on just about any capable computer system. For example a Core 2 Duo based PC seems to work fine as a DLNA server.
The first thing to do is to setup a DLNA server. There are several.
The PS3 Media Server project is a free and capable DLNA server.
There are several and all are fairly easy to use.
The only thing you need to do is to specify which folders on your system the server should serve via DLNA. Restart the server once make any change.
Then on your Smart TV or DLNA compliant media streaming box such as the Western Digital Live or Roku 3 search for a media server. Many people media streaming boxes, in this case your Smart TV does not need to be Smart or have DLNA because the media server box will take care of the DLNA component.
You should be able to now see the system name on which you have the DLNA server running and you should be able to browser content.
Many routers now come with DLNA built in. So you can connect a hard drive to a router and the router will act as the DLNA server.
For example the Linksys Smart Wi-Fi Router EA4500.
I find DLNA especially useful for pulling up video in conference rooms when making a presentation.