My first mountain bike in 1986 was rigid as is my current one. Ignoring changes in dimensions and angles the three things I’d say make the biggest difference to enjoying riding it are
Index gearing
Disc Brakes
Tubeless tyres for the low pressure rather than punctures.
I’m sure a huge part of the difference is down to changes in geometry and plus tyres but they are more part of a continual development rather than the introduction of a whole new way something works.
I’ve never had a dropper post so that might well be fighting for a place if I had tried one but given it’s all wheels on the ground xc riding for me then it’s debatable. I should also point out that all three of the above had been around for years by the time I got round to using them so were all well developed and working very well.