[will0]

Without a polarised plug it means current can 'flow' the 'wrong' way through the toaster, backwards through the heating coils.

Which is to say when the toaster is plugged in but off, the heating coils could still be live, and touching them would shock you.

[t0mas88]

Ah, European standard plugs don't have that. I guess that makes it mandatory to have the power switch interrupt both wires, at least that's what lamps here tend to have.