I was surprised to find Julia uses the multiplication method for string concatenation. For me it feels as wrong as using bitshift operators for I/O but maybe I'm missing something.
Multiplication for concatenation makes perfect sense: it is commutative, associative, and consistent with mathematical notation. A space would be even better. And the empty string would be great, but maybe difficult to implement if you want to allow multiple-letter variable names, as julia seems to do.
Even if using multiplication to join strings may be consistent with some properties of mathematical notation, it's less consistent with the usual meaning of the words (I checked the cosine similarity using GloVe 6B words embeddings, the similarity between "add" and "join" is 0.4 while between "multiply" and "join" is only 0.03), so it's probably the balance between being the general purpose and math oriented language.