An amazing look at the road to hell being paved with good intentions and an exploration of one possible answer to the perennial question of why superheroes don't ever do anything proactive to actually change the world and make it a better place.
A year later (before the Squadron Supreme mini was even done, really), Watchmen started exploring that same sort of deconstructive take, and it soon blossomed into a fad, in books and settings like the Stormwatch 'Change or Die' storyline that led to the formation of the Authority, the Ultimates universe (particularly the Ultimate Avengers) and even the Civil War (which also dealt with superhumans dragged into a more realpolitick situation) and the latest Phoenix Five business in AvX, which, once again, had heroes try to make the world a better place, and it all going terribly horribly wrong (because, gosh, it's been 40 years now, why would we want to tell a different story?).
The Squadron Supreme storyline proved that there was an audience for this sort of thing, and it could be argued that many stories that followed in it's footsteps were more amazing or thoughtful or whatever, but, if you want to use the word 'groundbreaking,' this was the one that really started to break that ground. It wasn't the newest idea on the block (back in the '70s, DC published some rationale for why the Justice Society didn't fly over to Germany and end WW2 by punching Hitler in the face, for instance, and I'm sure *hundreds* of comics have had storylines based on why character X doesn't fix real world problem Y), but it was one of the first twelve-issue storylines devoted to exploring that concept in depth.
And it brought us a whole new generation of heroes, to the original 'Squadron Sinister,' many of them not at all the sort of 'Justice League clones' that had come before, like Lamprey and Quagmire and Foxfire and Inertia, some of whom had vastly more creative and original powers than 90% of what we've seen in recent years.
Inertia just has the most awesome power you could have, in a world where people solve problems by punching other people really hard, the ability to absorb and redirect kinetic energy at a range, so that one super-dude can be punching the crap out of another dude, and she can steal all that energy, so that super-dude is batting at his foe like a playful kitten, and redirect that energy to use his own strength to beat the crap out of one of his own allies. So, so, brutally unfair! It's always fun to see someone use a power in a clever way, and Inertia did not disappoint in that fight.