As others have mentioned, the websites tend to be limited both by resolution and functionality.

My TV supports CEC(most do these days) which will pass the remote input onto the devices connected to it, like a computer. Which means with Plasma Big Picture I can navigate with my remote, and any app that supports navigation with simple arrow key input would work great.

Unfortunately, the streaming websites, last time I tried, absolutely suck at that and assume you are navigating with a mouse.