The differences between GNU and UNIX behavior can be substantial. I used a SunOS system for a few terms in college; I used Debian to get work done, but for a few things we had to make sure our code compiled and ran on the SunOS system.

One of the first things I noticed in the short while before I put the GNU tools at the front of my path: the SunOS tools wanted all options before all other arguments. So if you have "ls something", and you hit up, space, -l, enter, (or "!! -l" if you prefer) then instead of the long listing you expected, you get the same short listing as before, along with an error like "-l, no such file or directory".

It's minor, but it's one of those things that adds up when you're used to more capable tools and find yourself in a less capable environment.

OpenBSD doesn't necessarily suffer from the same deficiencies (I certainly don't know if they have that one or not), but when you're used to coreutils, any other tools can be a shock, and not typically in a good way. The same goes for environments like busybox, but at least there it's for a good reason: size constraints.

I'd be curious to hear examples where the reverse is true: are there instances of the standard command-line tools available on other UNIXen being substantially better than the GNU userspace tools?