Where are you hearing this? The Nexus S can absolutely support video chat, it has a front-facing camera. Whether a video chat app supports the Nexus S is a different question. Google Talk is supposed to support video chat in Honeycomb, hopefully it will be released for Gingerbread and support the Nexus S as well.
The Nexus S can't take HD video due to software restrictions, from everything I've read (example). I think it just wasn't prioritized since the Nexus S is a "developer phone". (If you meant to ask if it can play HD video the answer is yes, depending on the format and such).