Pretty much what everyone had said. I play a lot of Darius and it comes down to his base damage being very high. His passive makes it very difficult to win trades that go on for awhile, and apprehend keeps most people from getting away.
The main "counter play" to Darius is waiting for him to miss a Q. In an all in fight at level 6 or something, if he missed his Q and is against someone with similar farm he will lose the trade and or the fight. Not only does he miss the damage from Q, which is substantial he misses a stack of his passive too, so it takes more time to stack up for the dunk. The passive stack is less of an issue than the missing Q damage, but it is a hindrance. So if you are laning against a Darius and you see him either whiff Q trying to harass or he uses it to farm you should immediately go in and fight him, but then back out before his Q is back up. Easier said than done I know, but he has no passive sustain an is easy to shove out of lane if he gets behind at all.
If you do happened to be ranged, if he whiffs E you should punish him hard as well. Darius' damage is scary, but if he can't hit you then he's not scary. He is also incredibly immobile so it is quite easy to kill him if he gets out of position when your jungler is around. Basically what I'm saying is that his damage is pretty nuts with no damage items, but he is fairly easy to push out of lane if you play around him.
All that being said, it is questionable whether someone should be able to do so much damage as full tank and have his counter play being him missing skills. He is a huge lane bully, but if you don't get ahead you aren't terribly exciting late game.