Darius' counter is as it always has been; he has no escapes and his damage is massively backloaded. If you focus him down quickly, his damage output is negligible; he'll probably be saving his ult for killing blows, so he'll basically muss your paint job and that's it. He also has no escapes to speak of, so he's more vulnerable to ganks than almost any other champion in LoL.
But if you let him build up Hemorrhage stacks on your entire team, don't focus fire him, and then wonder why you lose when he triple dunks you, well, you lost that fight before he started jumping up and down on your team, you just didn't know it yet.
I had a game playing Darius on TT today that I very, very nearly lost. And I only won, not because Darius is awesome, but because our Lee Sin carried hard. The enemy was Renekton, LeBlanc, and Orianna. I spent the entire game blowing up in the first seconds of any teamfight, long before I could start murdering people left and right. And I built tanky, but still died too quick to carry teamfights. So that's how you counter Darius.
If your team is a bunch of tanky bruisers who win fights slowly via grinding the enemy down, you lose to Darius because he hard-counters that sort of team. That's why he was awesome in the old TT, where bruisers reigned supreme, and that's why he's still awesome in the new TT, where bruisers are still awesome, just somewhat less so.