communication (Pony express in 1900 vs texting Fiji today),
transportation (a month to cross the US in 1900, to 3 hours today),
which nations are at the top (it wasn't the US in 1900),
global population (only 1.6 Billion souls in 1900, and only 78 million in the US... now 7 Billion globally, and 38 million in CA alone),
medicine (imagine surgery in 1900),
Our grandchildren (or great-grands, depending on your age now) will laugh at Terabytes, keyboards, cancer, power lines, microwave ovens, bullet trains, and HD/3D.
The Pony Express was gone by the time the Transcontinental Railroad was completed in 1869.
With trains, cross-country travel was reduced to a few days.