In classic minecraft there used to be a convenient command where you could type /cuboid or /z and then place or destroy two blocks, and it would fill the area in between. There were countless more server-side commands and mods that let you build large scale structures. Copy and pasting sections, building spheres or cylinders, making shapes hollow, etc. Flying was much easier and faster too, although it did required a mod.

Minecraft now has so many more features and block types. But I miss some of those features, and building large scale structures is much more difficult.