I think + for append is fine, it certainly seems to make sense to me that you get "helloworld" from "hello"+"world".

I think the confusing thing is the type coercion. Does it make sense that "2"+3 is "23" but 2+"3" is not 5?

    (3*4).toString() + "=" + 3.toString() + 4.toString())

would not be as surprising that the two sides are different.