The problem is that the quality of the Chromecast streamed video depends on whether it has partnered with Google. Video playing on your laptop can be streamed via Chromecast, but Tech Crunch says that the video isn’t always that great.
Video streaming quality is quite good (on par with what I get on my Xbox 360 or my Apple TV, at least) particularly when pulling from an app or website that’s been tailored for compatibility — so Netflix, Youtube, or Google Play, at the moment. If you’re using the Chromecast extension for Chrome on your laptop to project an otherwise incompatible video site (like Hulu or HBOGO), however, video quality can dump quite a bit depending on your setup. It’s using your laptop as a middle man to encode the video signal and broadcast it to the Chromecast, whereas the aforementioned compatible sites just send video straight to the dongle, mostly removing your laptop from the mix. When casting video tabs on a 2012 MacBook Air running on an 802.11n network, the framerate was noticeably lower and there were occasional audio syncing issues.