Subtle differences in the way everything was done. slight difference like: the max size of a udp packet was slightly smaller on hpux (or was it solaris) such that things would break in unexpected ways. The gzip or tar params where all slightly off... You got used to it but it was a pain.
This reminds me why linux was ascendant. I'm not sure its better but much more consistent.
EDIT: My information was obsolete – apparently some BSD’s reimplemented gzip. I stand corrected.
I read somewhere that Bill Gates said in the 80s that Unix has a versions/forks problem which will remain with it forever. If that is true then it's one bloody successful prediction.
One inconsistency I've been very annoyed by over the years is the differences in ps. I ended up making a .cshrc that contained aliases to work around them. I'd check which OS I was using and alias a command to call ps with the arguments I used regularly. I used to work often on Irix, Solaris, and Linux and it was a pain to remember which arguments I needed. This was especially true on ssh connections where I didn't always remember which OS a particular machine ran.
MATLAB R2013b on Mac OS tried to do something similar. However, it sometimes decided that my hostid was the MAC address of my Ethernet interface, and sometimes my wireless interface (in which case it would reject the license). In the end I resorted to obtaining two license files and manually switching them when necessary.
The problem is that SUS and POSIX only define a subset of what's wanted by most modern software and a subset of the tools people need to interacting with the system. It's a double-edged sword, though: if we always stuck to what was common to all Unix-like systems, we wouldn't have lots of important OS features.
Like seriously....It ain't even funny.
So in the end, the situation isn't much better, if the goal is to reach as much UNIX like systems as possible.
Once, I printed a lot of it on a large format printer so I would have it handy while administering a mixed Unix shop. HP/UX, IRIX, Solaris, Linux...
So this link does qualify as a sort of Rosetta Stone.
Take a look at this: https://en.wikipedia.org/wiki/Rosetta_Stone