I am not sure if this helps when teaching a basic PDE class, but this is certainly a useful understanding:

Elliptic problems can be interpreted via diffusion processes. The solution at a point $x$ can be written as expectation of the boundary condition at the (random) exit point for the diffusion emitted from $x$ and associated to the elliptic operator.

If the boundary is smooth, then as $x\to x_0\in\partial \Omega $ the exit distribution converges to the Dirac measure at $x_0$, hence regularity of the solution.

If the boundary is bad, then the diffusion initiated at a boundary point $x_0$ can, with positive probability, hit the boundary next time at a completely different place, and the exit distribution can be very far from the Dirac measure, hence there is a problem.

notrequire any assumptions on the boundary regularity of the domain or the solution. If, however, you want to study the regularity of the solutionatthe boundary, then assumptions are needed. If the boundary is smooth, then it can locally be transformed into the upper half space of $R^n$ and it is possible to define the tangential and normal derivatives of the solution at the boundary and study their properties using calculus. If the boundary is not smooth, then these are not well defined. – Deane Yang Dec 11 '11 at 16:47