The eText Engine was built with NeXTSTEP simply because it could not be written with any other commercially deployed application architecture. I'd love to, but I have a finite budget, a finite amount of time-til-graduation, and infintesmally small tolerance for boneheadedness.
Bundles Dynamic loading of objects, automagic management of application resources (images, strings)... Don't even start with me about .DLLs...
Protocols As far as I can tell, the theorists haven't caught up with this one. Inheritance has its limits, protocols break past them... A guaranteed interface, regardless of typing is a powerful organizing principle for large scale systems. Try to extend Oberon's lousy Text object to do this... I dare you!
AppKit
There isn't a better integrated suite of ``class-library'' objects, multimedia support, application integration, and cool out there. So much of the system ``just works'' that I'm absolutely shocked, shocked, that a system like eText could take 25,000 lines of code. Have I told you about the Text class? No? Well, come over here, my pretties... 
Community There isn't a finer community of computer users out there... They deserve a tool like this!
Research tools, such as the Andrew Toolkit come close, but can't be deployed with the total integration and ease-of-use afforded by NeXSTEP.
Of course, with the public release of eText5, it's now a ``research system''... with all of the attendant support and reliability issues...
Consider, too, that other ambitious projects are based on NeXTSTEP for similar ``it-just-can't-be-done'' reasons:
to comment on So I was reading about MediaView.... MediaView.app, the multimedia compound document editor that inspired eText, from Dick Phillips at Los Alamos National Laboratories.
to comment on So I was reading about Draw++.... Draw++.app, a compound-document extension to Draw.app from Patrick Heynen at Cornell's Program on Computer Graphics.
to comment on So I was reading about WavesWorld... WavesWorld, a interactive 3D modeling system. Geez, those four words do it such poor justice. Take a look at Michael B. Johnson's wizardry for yourself sometime... WW3DWell will be coming soon as a proposed Annotation.