The simplicity of the design of the Great Pyramid is astounding and genius. Furthermore, each piece of the pyramid is giant, and it is simply astounding that humans so long ago with no modern machinery or technology were able to move them. The design and sheer mass of the Great Pyramid together make it the greatest structure of all time.
The Great Pyramid is an amazing work of art, especially given it's age and the time in which it was built. To imagine that the people of those times were able to create such a masterpiece is almost beyond comprehension. There is no doubt in my mind that this is certainly one of the greatest structures ever built.
The Great Pyramid is the greatest structure ever built. This is the reason why it is that great. It is actually made up of 6.3 million tons of material. Compared to all the other churches and cathedrals in England it is way worth. It's base also covers 13 acres and is flat and level to within 1cm.
It's probably best to think of it as a man made mountain. There aren't many man made things that have the same scale as the pyramids. Its just impressive that the biggest pile of rocks was done by hand. Getting a satellite to space is literally just throwing a ball, but we couldn't throw a ball that far until the last few decades. Nor could we pool as much water as the great dams, line up as much steel as the long railroads, or dig as deep holes as the modern oil wells.
Certainly it is one of the most astounding and incredible structures ever built. A true wonder. Still, advances in technology over centuries have enabled humans to create far greater achievements than Pyramids. While incredibly advanced and great for its time, the Great Pyramid simply does not have the brilliance of many other structures of a more modern era.