I'm thinking about creating a graphics tech demo to show in job interviews, and I'm wondering what do tech demos generally include...
Should I simply implement known techniques with little adjustments to the level shown in the demo? Or should do tech demos usually contain new techniques?

What is expected from a tech demo in a job interview for game/graphics programmers? Share your thoughts.

I think you can benefit from both. A demo with a known technique shows precisely that, that you can implement a known technique (hopefully showing that you understand the math behind it). New techniques show that you keep up-to-date on graphics tech. That being said, sometimes companies want a code sample. So also make sure to have good code in the demo in case they ask for the source.