Desiv summed a lot of it up. For myself:
Datatypes: allowing old software and the OS to use the latest formats, great idea.
This is one I'm still astounded nobody seems to try to copy.
Fits a lot on screen: seems higher resolution than it is.
This is a big deal for me on my laptop. Configuring a Linux system to use equally little space for folders is possible, but it takes a lot of work and "unusual" settings. E.g. I had to install a separate program to get Nautilus - the Gnome file manager - to use a global menu instead of littering every folder window with a menu bar, since it doesn't have an option to turn the menu bar off. And by default it opens with a side bar, location bar, status bar, menu bar *per* folder/drawer. Then you need to pick a different window theme, and a bunch of other things.
On the upside, I now have a Linux desktop with sort-of halfway Workbench functionality + Ken's icons.
DosDrivers: A great way of adding new filesystems, again, easy to understand.
Linux and OS X has FUSE which gives pretty much the same thing, but it was a very long time coming.
WBstartup: no hunting around for what the hell is bogging your comp down on startup. Check WBstarup and S: and your done.
Most other OS's have a similar solution these days...
Apart from datatypes and AREXX, one of my favourites is assigns. You can halfway copy the concept with symlinks on Linux (but not multi-assigns), and could clone it fully with FUSE (implement a filesystem for assigns), but it's not the same. And special assigns like PROGDIR: as well makes a big difference.