If one defines UNIX as POSIX, then, for the most part, or at least a pretty good part, Linux IS UNIX, because Linux implements much of POSIX, and in some cases does it better, e.g., certain functions for which POSIX is quiet or negative about thread safety are thread-safe on Linux, and there are almost always TS versions, e.g., strtok_r.
Outside of POSIX, one has to conside OS specific features, two of which come to mind as superior, though this is a judgement call: Solaris Zones and Mac OS.
I was a Linux desktop user for years, but I found that over time the desktop just started getting in my way (this was tennish years ago, so I am sure things have changed). I’d curse how gnome or KDE had once again broken my UX, then I’d help my wife or daughter with their Macs, enjoy the experience, the curse when I got back to my machine.
One day, after a particularly frustrating battle with whatever desktop I had then (I’d gone from Ubuntu to Mint to Debian to I cannot remember what, trying to find the sweet spot), I cursed, yelled that I would be back in 90 minutes, and bought my first Mac.
They’ve been a joy to use since. If Apple does something stupid, e.g., stopping at an old, brain dead bash, well, homebrew, et al, to the rescue.
Solaris Zones were wicked. One could get full MAC with labelled networking and restricted root with ease.
It’s finally possible to get close to Zones with a mix of capability management, namespace management, SELinux, et al, but not as easily.
So, yeah, there are individual UNIXisms that are better than Linux, but overall, at least on the server side, Linux is UNIX improved.
XFCE is one of my own fallbacks (I usually prefer WindowMaker), though fvwm, twm(!), the boxes (open-, black-, flux-, hacked-, etc.), tiled WMs, etc., are all perfectly serviceable.
Some may look* vaguely dated, but tend to be rock stable and blazingly fast.
XWinMan is still live, I find: <http://www.xwinman.org/>
When I started college in 1993 our default environment was mwm but I quickly switched to vtwm and had a really cool setup. I switched to AfterStep because I loved the NeXTSTEP look and used that and WindowMaker for 10 years before moving to XFCE.
ZFS is an example of one that's recognizable, you can bolt it onto linux and it (now) works quite well, but it was integrated much more into Solaris.
There are also Unix platforms that fully support hot-swappable just about everything, which you CAN do with Linux but it's not as simple.
Most of that stuff is gone by the wayside now, Linux is "good enough".
It won't surprise me if one of these days Oracle decides that Canonical is a ripe lawsuit target over ZFS.
It's similar to the "polish" available on macOS or Windows, Linux has all the pieces but you have to assemble them yourselves, even today.
Because there's a moderate change that it goes against them.
I hope btrfs becomes quite stable and usable.
That whole CDDL/GPL incompatibility is BS.
(speaking as the guy who wrote the scripts to change all the license text, though the actual relicensing was due to a lot of work on my boss's part.)
And canonical includes ZFS, and i think they have more lawyers then you and me.
Do you really think they would take that risk especially with oracle to include ZFS (that is not even their main fs)?
Have some commonsense.
back when, unices were better at real multiuser, while linux was much faster at all sorts of I/O and CPU bound things in the personal-computer, single login context. Like massaging massive amounts text data (which everybody was doing to webify it), linux was blindingly fast, but don't trying to do much else at the same time, it would bring your machine to its knees.