Basically Freesat offers all of the channels that you get via Freeview, along with Freesat exclusive content, which includes high definition channels. At the moment, HD content is limited to BBC HD, although as already mentioned, ITV HD will be arriving soon. An interesting feature that's due to arrive with ITV HD is dynamic switching between SD and HD versions of the channel. Whereas, say, Sky One and Sky One HD are completely different channels that show identical content - one in SD and one in HD - ITV HD is set to be a single channel that can switch between SD and HD. So, if you're watching ITV on Freesat and an programme starts that's available in HD, you can press a button on the remote and you will be dynamically switched to the HD feed, then switched back when the content is SD once more. This is a neat trick, but hopefully one that won't be needed for too long, since we'd obviously like to be watching everything in HD eventually.
It's important to remember that Freesat offers no quality advantage over Freeview when watching standard definition content. Of course what it does offer is clean reception for people who can't receive Freeview - I only live in Surrey, but even I need a serious aerial, on a pretty tall pole on top of my house to receive Freeview clearly, while others in the office simply can't get it at all.
Going back to the Ethernet port that I mentioned earlier, it doesn't take a genius to figure out that with the BBC so heavily behind Freesat, Ethernet connection makes for a simple way to view the BBC's iPlayer directly on your TV. The fact that an Ethernet port is a mandatory feature on a Freesat branded product, makes it clear that iPlayer is part of the strategy. Although Freesat has yet to officially announce iPlayer support, I have had confirmation from a number of sources, that this will be coming later in the year.
Of course having an Ethernet port built into a TV brings with it all sorts of other advantages, as seen in the Philips Cineos 37PF9731D that I reviewed a couple of years ago. The Ethernet port in that Philips TV allowed you to stream video from external sources directly to the TV, with support built-in for DivX and Xvid. Unfortunately there's no support for video streaming via the Ethernet port in the TH-46PZ81, and no built-in video codec support, but Panasonic did say that this feature could well appear in the next generation of products.