1. Indexed shifting. Most of us probably would not be riding MTB if we had to feel the gear changes manually with a friction system. Indexed gears made gear changing gears so much easier, almost second nature.
2. MTB Specific (Compact) Cranks and cassette. Initially “Micro Drive” 22-32-44 with a 12-32 cassette. Again, a lot of us would not be riding trails if we had to use borrowed touring gears in the 24-34-46 and a 12-28 cassette as the widest sperad. The lower gears made MTB accessible to average recreational riders, and cut the number of hike-a-bikes substantially.
3. Suspension. A skill compensator maybe, but makes riding much less fatiguing and more enjoyable so easily adopted by average folks. Most average recreational riders would be put off of MTB if rigid was the only option.
4th would be MTB specific tires, but the OP only asked for 3.
