To me, D&D is the foundation of modern gaming. It has every aspect of history of gaming and the Human Condition wrapped within it. Its a perfect hologram of our psyche. Chance, Skills, Identity, Idiology, Politics, Morality, Corruption, Evil, Good... all the sins and mythos and imaginations... Its wonderful - and the world would not be the same without it and all its ilk. I do wonder what Japanese equivalent may have…
The combination of D&D and western RPGs with Japanese games/comics/animation has been amazingly fruitful.
I believe most of the familiar tropes (character sheets, experience points and leveling up, hit points, classic party composition, etc.) were originally imported from D&D, possibly with a parallel path through early PC RPGs and adventure games. Dragon Quest and Final Fantasy both had levels and HP (and I believe those games were influenced by Wizardry and Ultima, in turn also influenced by D&D.)
One thing I don't know is how popular D&D and other TTRPGs have been in Japan.
MUDs were definitely influenced by D&D (as well as early CRPGs and adventure games) and evolved into MMOs.