Other than watching movies the most communal entertainment experience would be watching sports. You can watch college football with all of your buddies from around the country at once. I've only used the Party view once, it was fun, but they need to create some kind of speaker phone system so you can literally link one actual party to another.
I think though this speaks to a larger goal of Microsoft in the living room. I think eventually the model of having every single channel, even channels you don't want beamed to your tv may be coming to an end. The model of the future will be ala carte channels and shows you choose, automatically downloaded to your box, which limited embedded advertising. This is already the model that MS uses for Zune, I think the next step will be TV and Video Entertainment. A flat monthly fee, unlimited downloads... The only hurdle are the Studios and the various Unions have to come to a true agreement on each other share of the pie. So don't hold your breath...