One of the best things about macOS is the special character inputs, which is one place I think Apple has outshone their competitors for a while. Even if Elementary doesn’t introduce the same keybindings it would be nice to have some set up automatically without having to fiddle with the compose key settings. It’s a very common UX issue imho and also an area where you can easily “beat Windows”
I don't know if it's what the OP meant, but on macOS you can hold a letter key, and it will pop up a little dialog with a bunch of similar characters (hold 'n' and chose from 'n' with various diacritical marks), and you can just press the corresponding number key to choose it. I'ts not as fast as a dedicated key shortcut if you're constantly using the same character, but it's far easier if you're only occasionally using various characters and don't want to remember different shortcuts.
Just tested this - it seems like a handy feature but apparently also dependent on the application using a specific textbox from the macOS UI toolkit. So it doesn't work in iTerm, and probably not in Firefox either? So not an "OS" feature exactly, and probably something the GTK folks could provide an equivalent for if they don't already.
On macOS you can also type things like Opt-U to get an umlaut, followed by typing the letter you want underneath it (eg for ü, ë, ä, ö). ß is Opt-S, I think é is Opt-E E, etc. I prefer that to the AltGr+key shortcuts, which I find harder to remember. I don't use the Mac as my primary OS anymore, but that's one of the things I miss the most.
I'm not sure what GP means by special characters, but I use fedora workstation 30 for my day to day life and I can attest that Arabic and Chinese input are absolute garbage compared to OS X.
Arabic key layout is different (which fine, OS X uses something non-standard anyways), but makes it absolutely impossible to customize it. Text boxes frequently jumble themselves up with left/right mixed input, while OS X handles this as expected.
Chinese input is slightly better, as you don't have to deal with order wonkiness. But pinyin input is halfhearted at best, and requires a bunch of qol changes.
Mostly the Latin-1 subset (plus East Asian overlines and the Chinese ‘v’ tone mark) and other characters that appear in names, but also “”‘’–•—¿¡§ are all rather useful if you can get to them. The math symbols for partial, adjoint, sqrt, almost equal, not equal, equivalent, isomorphic, geq and leq are also nice; others like sum and integral are pretty but require too many decorations to be useful anyway. On OS X last time I used it if you press eg Option-u you get a hanging diaeresis and the next character you type will be umlautted if possible. You can enable this on X with [Compose]+:, but you usually have to fiddle with settings to enable it, and that’s not the Elementary philosophy.
Too often I want to use the proper spelling of someone’s name and I run off to Google to get the symbol.