If you use a trailing slash on the source it copies from the directory, if you omit the trailing slash it copies the directory itself. AFAIK this is pretty standard across POSIX tools
the trailing slash is pretty convenient.
I think trailing / could be a nice way to indicate some meaningful difference, but since autocomplete always sticks it in, just feels like a bad idea to me. I might like it if directory names always had to have a trailing /, but I am less motivated by "convenience of common cases" and much more by "absolute precision/specificity/unambiguity" belt and suspenders.
(kind of unrelated but along the same lines, I toy with the idea of getting rid of . and .. visible in the filesystem, and make them only part of the syntax of paths. then you could have unambiguous multiple links to a directory: ".. is where you came from" and .. in the root is still the root, so chroot works too)