I like the Circle-Ellipse Problem [0] as an example of something CLOS / multimethods are good at that C++ isn't. In general, Expression Problem-like [1, 2] problems are much much nicer to solve in CL.

[0]: https://en.wikipedia.org/wiki/Circle%E2%80%93ellipse_problem [1]: https://en.wikipedia.org/wiki/Expression_problem [2]: https://wiki.c2.com/?ExpressionProblem