sed
* The `-i` flag - In the absence of a file extension given, I should not have to specify with `''` that I want the original file over-written. The flag is called in-place for a reason.
* BSD sed doesn't support ANSI-C escape sequences, so you have to fall back to your shell quoting them for you.
xargs
* Why is there no `-d` flag for BSD?
There are others I've found over the years, but those come to mind as annoyances.