Instead of being an oddball power which you had to luck into, and which a lot of house rules just ignored, everyone had some psionic ability. There were specialized psionicists as well, but it was just a world where sentient beings (being D&D there were some racial exceptions) had psionic power.
I was looking through my old books recently. The player rules for Planescape, another massively different campaign setting, was ~30 pages. Some of that was full page art, and they dedicated one page to each of the 16 factions (including non-factioned, IIRC). With all the jargon, lingo, and weirdness in Planescape, it was almost all lore changes. Under the hood, it was still the same old AD&D.
In the first Dark Sun Campaign Setting box set, you get two main books: one book of lore and one book of mechanical changes. Each book is ~120 pages. There are rules for new races, new classes, new weapons, new armor, new spells, new environmental hazards, etc. The setting was 50% new lore and 50% new and changed mechanics. And DSCS didn't even have rules for psionics in it! They had a couple pages detailing the new psionic character creation rules, but it literally tells you to use the Complete Psionics Handbook! (The second Dark Sun Campaign Setting included a ~90 page psionics book that includes most of that content.)
To be fair, I have never looked at Dark Sun.
I think psionics could be workable, in a certain light, but that's the subject of a rather half-baked and long essay series I have been kicking about for a decade as to how to "fix" many of the issues in D&D in a way that does not result in the Endless Next Edition problem.