The appeal to me is that its simple, and that you can browse from your laptop, tablet, or phone. I (and I think many others) prefer browsing content on those devices, versus, say, an xbox using a controller that was designed for games (or a remote control accessory, which would cost almost as much as chromecast itself. Also, the xbox live gold requirement on xbox for a lot of the content is enough to keep myself and many others away from streaming via xbox.
I have an xbox hooked up to my tv, but I rarely use it because i have an old laptop with an external tv tuner hooked up to it that I use for watching everything online and over the air. I also have my phone and ideapad yoga on the coffee table. Unfortunately the old laptop's days are numbered I think, so it might need to be replaced. However, if chromecast works well enough, I could just use the yoga to stream everything I want over to the tv. It would also let me get rid of the dreaded keyboard and mouse on the coffee table.
My worries though are that the netflix app on windows phone won't be included (although I guess I should be able to use the yoga to send netflix to chromecast). I'm also wondering if support will be integrated into windows-desktop audio/video apps (like WMP, VLC, etc.), in order to send local content to chromecast.
I hope chromecast turns into what I wanted xbox smartglass to be. I don't use smartglass often because I can't send locally downloaded content to the tv.