I've been playing around with the "Friends Edition" of Joost, and I'm really impressed. The focus on the full-screen viewing experience is what I noticed first, and it really sets it apart from in-browser based video. When I launched it, I felt myself relaxing -- which is what I want to do when I'm watching TV. I don't want to see all the other crap on my computer that is trying to get my attention. Thus the overall tone the application sets is one of entertainment. Overall it is very Tivo-esque, even thought the interface looks nothing like Tivo. (FYI - drop me an email if you want an invite to Joost - you can find my email address in the sidebar.)
It is built on the Mozilla platform ("heavily-modified", according to NewTeeVee). From my own experience developing an eBay toolbar for Firefox, I know that the Mozilla platform is really powerful, but I ran into problems because I don't have C++ experience, and many of the things I wanted to do required that I build XPCOM code. I read this morning that Silverlight is based on SVG technology, and apparently Joost is too. Seems like I should get familiar with it, and if you are reading this, you probably should as well.
My only complaint is that when I tested with an earlier version (0.8), it wouldn't work from within my companies firewall (unlike Skype, which works perfectly). Not that I wanted to watch TV at work, I was just, um, testing it... Not sure if this is still a problem in the new version.
The video quality is excellent, which leads me to my point about AppleTV. Apple has completely dropped the ball with the video quality in AppleTV. I was in the Valley Fair Apple Store the other day, where they have 3 42 inch plasmas set up demoing the AppleTV. I overheard two different couples who, after looking at the demo of "The Incredibles" said almost exactly the same thing: "It looks really pixelated." Of course it is not the AppleTV itself that is really at fault - it can display much higher quality content, it is just that the movies and shows available from the iTunes Music Store aren't available in a high-quality format. Apple's strategy is that providing an integrated solution, but the quality of the solution is only as good as the weakest link, and that link is, for now, pretty darn bad.