Not that it was ever particularly difficult relative to other games, but in the original game and TBC, levelers often found themselves in dicey situations that kept things from getting too dull. In WotLK and Cata, baseline player power saw a steep increase and the world was largely defanged, making it so levelers rarely faced significant danger.

It's moreso that you were forced to find friends back in the day. Vanilla WoW was still pretty similar to that, but the older games like FFXI and Everquest required that you team up in a party to level at all. It led to a completely different experience, where a server was a community of people who you knew and played with for years. There were repuations, drama, politics, player run events, infamous trolls; everything that no longer exists in MMOs that actually made them great and not just multiplayer games with a big lobby.