I've been using Boxee for all of ten minutes. However, I can already tell you that one of my favorite features is the support for Internet video sources. We have a mac mini connected to the living room TV, and we mostly use it to watch Internet video, DVDs, etc. Browsing YouTube with a mouse and keyboard in Firefox on a TV is not a good experience. Browsing YouTube with boxee is wonderful.
However, because Boxee's support for Internet sources is limited, I know we would have to keep going back to the browser. We frequently watch videos from gametrailers, vimeo, escapist magazine, college humor, hulu, South Park Studios, and other sources. I know there are technological difficulties in supporting some of these sources, like hulu, but others are very possible, like vimeo.
The best feature boxee can possibly add, for us, would be for it to support every Internet video source we use, so that we would never have to use the web browser to watch video in the living room.