Leveling in vanilla was both a bit more difficult and annoying. I don't know why these arguments are always framed as mutually exclusive.
It was true that you could die more easily if you messed up or pulled things in the wrong order or generally weren't paying attention. I don't know if having to pay attention makes the game "harder" as such but fine. It's not worth arguing about. You did have to pay more attention then and do more personal resource management when leveling. You don't now.
It was also true that the game was much more fiddly, OCD friendly, and prone to make you do pointless things all in the name of "realism". I'm of two minds about that. Some of it wasn't really necessary to have a proper RPG experience and some of it was. Doesn't matter either really because most of it was eliminated in favor of 1) moving the story along and 2) moving you along so you could raid. I don't know that the first was necessary but I do believe that the second thing pretty much gutted the world game for years.
Legion had Suramar which was a decent attempt to get at least a bit back to that but I think the game could use a lot more.