Tied up with PnP, PCI has to be a big one. It was the advent of PCI that finally did away with hours of setting jumpers on cards to configure IRQ, DMA, base address and so on. I worked in support before then and it was a bloody nightmare helping folk do that remotely, especially when everything was an add-on board rather than integrated into the motherboard. Sound card, graphics card, network card, modem, serial ports, parallel port, game port all needed frobbing manually to avoid clashes with each other. The fact that we’re still using a variant of PCI today says it all.
Similarly, and it pains me to say it because it was wildly unpopular at the time, but Windows 95 / DirectX / “Games for Windows” was a paradigm shift. Prior to then, every new game you’d buy would require endless fiddling with config.sys / autoexec.bat and memory managers to squeeze out the last few drops of conventional memory in order to satisfy whatever specific requirements that particular title had.