Edited for Syndication: To allow for longer commercial breaks (and in recent years, to make room for Pixar's Toy Story of Terror!), ABC typically cuts a few scenes such as Lucy challenging Charlie Brown to kick the football and Schroeder playing the medley of WWI songs for Snoopy.
Franchise Killer: Subverted. CBS warned the crew that if this special failed to generate ratings as high as those received for A Charlie Brown Christmas less than a year earlier, they would not commission any more Peanuts cartoons. It ultimately did so well, new Peanuts cartoons continued to air throughout the following decades, even after Charles Schulz's death.
The network literally said it had to be a "blockbuster". It ended up tying for the #1 most watched program of the week with Bonanza, which was a ratings GIANT for most of its run (including the year Great Pumpkin first aired).
Long-Runners: It's only one special, but considering how many consecutive years it's aired on TV? It definitely counts.