People often confuse difficulty with being time consuming. Leveling was slightly more difficult in vanilla, instanced content not really. Most of the difficulty in vanilla relates to how time consuming the preparation was. Mechanically it wasn't very difficult.
I do think WoW has made a mistake with the super accelerated leveling in the game, and the speed at which you can do things now, it makes it all feel trivial. It took a lot of time to get a full resist set together in vanilla, if you need to pick up gear to run some specific content now it doesn't take very long at all.