It's actually huge. You have to take into account that before entering noxian might, Darius's damage is still scary but not particularly threatening at all for a low-utility slug. Darius is a champion known for doing his best to try and prolong a fight to noxian might, when he can finally pop off.
These "loves taps" as some in the community call it are actually hitting his highest moments in stages of the game that really matter to darius the most (early, mid, and the transition from mid to late, before late game where he starts to fall off to mediocre).
If you look at the graph, noxian might doesn't come back online to live values until level 14 in the game, where darius soon begins to fall off when ADCs build LW.
What does this mean?
Darius has 5 AD scalings. His passive, his Q, his W, his R, and his auto attacks (duh). Nerfing his noxian might early indirectly affects almost his entire kit when he finally is able to achieve noxian might on an enemy champion for the early and mid game.
This nerf is huge, and personally I believe it is unwarranted. No one in the darius main community minds the Q hilt nerfs, because that provides the "hug him" counterplay avenue with a little more reward and keeps darius's reward pristine if he lands the outer edge. However, the passive nerf is a straight up nerf to darius in general, and as the graph shows, it actually does hit harder than a lot of people realize. His five stack dunk will be weaker, his Qs, his Ws, his auto attacks, and his E armor pen won't mean as much as it used to with lesser AD.
Darius hasn't received a direct nerf or buff since the later end of season 5. It's obvious to any rational human being that the meta shift caused by jungle changes and ESPECIALLY the removal of SoTA for courage of the colossus is what caused darius to climb where he is now. After the first round of Colossus nerfs, darius is sitting at around a 52% win rate with a DOWNWARD TRAJECTORY.
Darius's most common keystone is CoC, and it's also his highest win rate keystone. Courage has some pretty big nerfs on the PBE, admittedly almost on the extreme side according to Riot's own SmashGizmo who said, "Uh, so Courage of the Colossus changes that show up on PBE soon might look a little harsh at first, starting at the extreme end here i think." Darius is about to get nerfed on two fronts, and I don't think that's right. We should wait to see after Courage gets nerfed again to see where darius will land.
KEEP IN MIND
Before courage was introduced, darius sat at LITERALLY A NEAR PERFECT 50% WIN RATE WITH THE SAME EXACT KIT AND NUMBERS HE HAS RIGHT NOW. When Courage gets toned down to the point where darius can't really abuse it anymore (it'll still be a good keystone, just not so oppressive, and darius can take other keystones when he doesn't feel the need to go with CoC), he'll most likely revert back to where he was before this whole mess happened, a near perfect 50% win rate.
Before CoC most people on these boards praised Darius's design for being well rounded with clear strengths and weaknesses, a poster child for the juggernaut update compared to messes like mordekaiser and Skarner.
I ask that Riot think through what they're about to do. I would advise waiting to see how CoC nerfs would affect darius.