The entire premise of Portal is based on the ability to use 3D rendering technology to create the effect of opening up portals between surfaces. I don't think anyone would argue that Portal is a shallow tech demo. Narbacular Drop maybe was a shallow tech demo, but that same tech-based premise, when used by a masterful game design team, created one of the best video games of all time.

Likewise, the entire premise of Minecraft depends purely on the simplicity of a voxel-based world - a technique for defining a procedurally-generatable, deformable landscape. Maybe Minecraft is only a tech demo or an interactive experience, but... it's a damned successful one.