Some really basic concepts weren't even realized back then one big one is hypertext - its not really a complex thing when you think about it but undiscovered then.

At the time many folks were graduating from cassette/paper tape to floppy disk, so random text file usage was kind of a new thing to many micro owners, text files were there but mainly used to store program variable data. Other concepts like compression, graphics formats, remote updating, distributed data, etc. were in their infancy.

I would think someone going back there would have an advantage, they may have to learn the old languages then build a whole lot of tools, but they would know more advanced end results which is a lot better then trial and error to reach those points originally.

Hypertext was in the Mother of All Demos in 1968:

https://en.wikipedia.org/wiki/The_Mother_of_All_Demos

I think it would be difficult for anyone that currently develops software for the web to truly get that whole era. 4k of memory.... 4k. I have people telling me that I'm prematurely optimizing things all the time. "Memory is cheap". If they knew they had 4k, every step of the way would be an optimization. All variables would be named a letter. No garbage collection because we'd be mallocing every single byte and mfreeing it asap. I really am wracking my brain to think of a concept that we have that we could have had in the 80's. Most of the greatest things are because we had CPU cycles and memory to burn.

Toys - a few years back some toy manufacturer was looking for some 6502 coders to work on a toy, had gobs of ROM storage (for code and audio) but only 256 bytes of RAM to work with.

Many of the early systems were made in a very short amount of time by a very small number of people who had remarkably bad tools.

I wrote code (pre-STL) for linked lists, sorted arrays, array lookups, and everything. And without modern languages (like C#!), I had to fight with not screwing up pointers.