I remember Frontierland as swarming with activity during the daytime, but quiet at night. Mine Train, Pack Mules, Canoes, Keel Boats, Mark Twain, Columbia, Golden Horseshoe Review, fake shootouts, running all over Tom Sawyer Island and Fort Wilderness. The ultimate playground for kids. It wasn't just the specific attractions but also the overall feeling of being there.
At night, though, Tomorrowland was the happening place, with a shiny glamour and constant motion everywhere - PeopleMover, Rocket Jets, Monorail, Skyway, Submarines, even a couple of buildings moved (Tomorrowland Terrace and Carousel building). Everything in motion and a sense of glamour and excitement. The attractions were good but the overall feeling of being there was more important.
Now both lands are just collections of attractions. The feeling of being there is long gone. The people who argue endlessly on this forum that it's "just nostalgia" and that the park is actually better today due to the newer attractions, bla bla bla, aren't taking into account the intangible qualities, especially how you felt when being there.