What would a rock thin section look like without any polarizers?
It would look almost identical to the image obtained using plane-polarised light.

Some minerals would look darker, especially those with strong pleochroism such as biotites and amphiboles, because all light is now transmitted or blocked regardless of orientation.
Minerals with large relief differences such as carbonates (e.g. calcite) would cease to have the differences and would just have high relief.

From a static image it could actually be hard to tell that there is no polariser. A careful and experienced observer might deduce it from small hints, given some prior knowledge on what the rock contains.