What can be said that hasn't been said hundreds of times before by those of us lucky enough to experience it in all of its glory.
This was the most inspirational, awe-inspiring, futuristic, yet, realistic...the amount of scientific realism that went into it was obvious. It wasn't a pipe dream or mere fantasy, it was a true portrayal of potential life in the coming decades and centuries...if we as humans wouldn't lose sight of those visions.
Sadly, on so many more levels than the loss we experience when other classics closed, like Mr. Toad or 20,000 Leauges, this meant more as a real testament to the future of technology and ideal living.
Will we ever see anything like it again? It is possible...because if we can dream it we can do it.
Some other people have said that if Disney ever decided to close the Carousel of Progress, then it should be moved to the Smithsonian. I agree with this, and since Horizons is directly related, I think it should also be re-built at the Smithsonian somehow.