10% of this is knowing in advance what UI/UX paradigms you're matching/mimicking.
The original developers had to come up with the concept of, say, a taskbar from scratch.
They understood it has to run separately from other parts of the OS in case of crashes, it should support icons that are references to the same icons that windows have, etc.
But they were also limited by the resources of their day, and had to get clever about resource allocation. That's extremely time-consuming.
But that said, 90% of this project is simply a very good creator, being incredibly productive.
You are absolutely right about the benefits of mimicking existing UI paradigms. It saves so much time already knowing basically how I want things to look and work. And not having to worry as much about resources as people did in the past is a huge load off. (Although I do all my development with 128MB of RAM or less, because I enjoy that sort of challenge..)
Serenity is an unapologetic remix/mash-up of all the things I like in software. Not just visually, but also code-wise. If you look into the sources, you'll find C++ heavily inspired by Qt and WebKit.
The point is that the device with 2KB RAM is a single-purpose embedded controller, and the device with 4MB RAM used to be a full-blown universal desktop daily productivity machine.
Well, the first computer on which I wrote programs was a ZX-81[1]. The 486 DX4-75[2] with 4 Mb memory, however, helped me learn C using DJGPP and later build my first Linux kernel (on Debian 1.1), browse Gopher and WWW sites, read newsgroups, play with Cello[3]. I also wrote first versions of my economics experiments on it. Later, I upgraded to Windows 95 from Windows 3.11 WfW so I could have a decent Java experience.
I don't think I have much of a point except that you could do development on that machine in relatively modern ways in a way I could never imagine on a ZX-81 or ZX Spectrum.
Anything is possible! It would take quite a lot of work to bring up a Qt port, so it's not really a priority right now as there are many other important things that need attention. But some day.
The original developers had to come up with the concept of, say, a taskbar from scratch.
They understood it has to run separately from other parts of the OS in case of crashes, it should support icons that are references to the same icons that windows have, etc.
But they were also limited by the resources of their day, and had to get clever about resource allocation. That's extremely time-consuming.
But that said, 90% of this project is simply a very good creator, being incredibly productive.
The mythical 100x programmer isn't always a myth.
I'm in awe.