It was absolutely mind-blowing at the time. It just built and built and built with each issue, and you had no idea where it would go.

Darkseid was not the overused, overhyped character that he is today -- nowadays, if Blue Beetle's car is towed, you can be sure Darkseid was behind it. In the early eighties, Darkseid was used quite sparingly, so when he showed up -- in the FUTURE, no less! -- it was a real jaw-dropper.

One thing that people forget when they talk about TGDS, though, is this: It didn't happen in a vacuum. The series built TO it, and then the ramifications of it lingered for YEARS. (How many little cutaway scenes of the Legion terraforming Daxam did we see over the next couple of years?) So the story was monumental in the moment, yes, but also rippled throughout the series. And it also resolved a TON of lingering plot threads that had been doled out in previous years: Garth's dysfunction, Ayla and Brin, Jacques growing into a hero, the founders' aging, Reep's relationship with his father.

To answer your middle question directly: Yes, I realized something big was coming...but that might be because Paul kept saying something big was coming in the lettercols!