> everyone seems to have a different perspective on what D&D even is.
There's a variety of axis you could split it on, but if you wanted to look at the departure from low-fantasy adventurer's just trying to eek out an existence to the players becoming heroes going on epic quests, there's a pretty universally agreed upon point that really kicked this shift into overdrive, and it's the original Dragonlance modules. They weren't the first to position the game this way, but they were the first to really stick with the population. We started seeing more AD&D splatbooks that raised the power level of characters, the Companion rules came out for the Basic/Expert line that raised the power levels there, etc. AD&D 2E further embraced this, though you could still play the more traditional sword and sorcery style, but 3E largely cut off the original style of play completely.
In the early days it was much more of a low fantasy environment. The epic high fantasy angle grew over time.