I disagree with you, and I don't have Darius face rolling match history;
Hell, I don't even OWN Darius.
I fail to see how a single spell, his Ult, is somehow this world changing event. This is an execution move, its designed to... execute. I've not seen a single instance of Darius using his Ult on someone 100% Healthy, and it instantly killing them. Never seen it happen.
I'm sure it CAN, but if you've allowed a Darius or -ANY- champion to be fed enough, that they can instantly CLEAVE YOU IN TWAIN with a single skill - nerfing the champion wont fix your issues.
Every champion can be dangerous, some are built to allow this in different ways, Darius is an example.
He has no escape mechanics, he has no gap closers - he has a pull, granted. But if this is missed, the CD seems to be an issue from what I've heard and seen in matches.
His Ult refreshes, this seems to be a sore subject for a lot of champions. Any champ that has a refresh mechanic tends to send the community into a frenzy. But it requires a kill.
Not an assist, a kill.
Darius feeds off of actual kills, if you shut him down - even in a teamfight, all he really has is Decimate. Thats it.
Decimate is his reliable teamfight ability.
Noxian Guillotine is dependent on either his TEAMS ability to whittle down YOURS, or the PLAYERS ability to focus down on YOU.