After the rules supplements Greyhawk and Blackmoor, we get the first one which is not titled after a setting, but promises to give us some wizardry stuff. While that’s not a complete lie, the truth is that what it is concerned about mostly is Psionics… and what a needlessly complicated thing that is. There’s a reason why Psionics did not stick around and that’s because it just adds another layer of complication to the rules that kind of make it very unwieldy. 

However, the supplement isn’t all about psionics, there are a bunch of things that did stick around the rules up until today, and particularly things about the lore of D&D that first appear here which are emblematic of the game’s universe. First you get the new class of Druid, together with Druidic spells, a lot of the new Psionic powers would also later become spells, so that’s a part that sticks around. 

In terms of lore you get a bunch of named monsters like Orcus or the Demogorgon that come straight out of Gygax’s Greyhawk setting and would still be showing up in Stranger Things in the 2020s. Speaking of Stranger Things you also get the first references to Vecna here, with item descriptions for the Hand and Eye of Vecna, two truly nasty artifacts that give great power at great cost. A mix of bad ideas (psionics) and great ideas (everything else), worth a look.