Lots and lots of transition animations, and potentially very poor UI/UX model.
Demos are not (largely) interactive, so the demoscene is not known for collective skill designing GUIs.
The most similar/related type of things I can think of that are interactive are crack/keygen programs that include chiptunes and maybe a cute graphic effect somewhere.
For some time I've actually wanted to go track down such programs to analyze/compare/study the progression of design. Kind of difficult to do so though, for obvious reasons (unfortunately).
But my impression from the one or two programs I do very vaguely remember using, half for entertainment purposes was that the UI design was sort of at the "48%-50%" mark - good enough, but flitting just below the halfway mark instead of just above it. Not remarkable; maybe a few rough edges.
Thanks for your comment. It prompted some mental gears to spin and for me to realize the above.
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I do think I understand the spirit of what you're getting at, though, and there are a few things that my brain has decided is relevant, although I can't describe the working-out process that led me to list these. ¯\_(ツ)_/¯
- SymbOS (https://en.wikipedia.org/wiki/SymbOS): preemptive multitasking, windowed GUI, storage on devices up to 128GB, runs on home computers using the Z80 (starting at 4MHz) with up to (_up to_) 1MB of RAM. (I found this rambling but very interesting demo of what it can do with one of the more capable machines it supports: https://www.youtube.com/watch?v=2-oBNh0UkQc)
- kOS (https://www.youtube.com/watch?v=kTrOg19gzP4): limited-access OS and brutalist GUI using the K functional programming language (hmm, I should watch this video again)
- KolibriOS/MenuetOS: you're likely already familiar with this; it's a pure-assembly-language GUI that runs on 32/64-bit x86 PCs
- AtheOS: Amiga/C64/etc inspired kernel + GUI + application suite, written (for x86) by one person over circa 10 years before it became boring; a small team picked it up a few years later and branded their continuation of the OS as Syllable, although the Syllable website seems to have died at some point
- Contiki: runs a GUI on a Commodore 64 within 30KB of RAM
- There is of course TempleOS, FWIW