Nobody is saying Darius as a champion is OP. People are complaining his Ult is OP. And it just plain is. Decent range, true damage, low cooldown, forgivable mana cost, refreshes, and doesn't consume his DoT.
His passive DoT is annoying since it can be built via skills and/or AA and scales with AD that does magic damage. If it was one or the other, not both, it would be more fair. Why does it need to scale? Why does it need to do magic damage? Why does he get to build stacks with AA and abilities?
It's not about poking Darius. It's about plain out-living or out-feeding him. You could poke him all day, but he has one of the highest Hp5s, has high HP, and high armor/mr. If he rolls cloth and 5 pots you're gonna be there a while.
Every champion (except maybe Olaf) is screwed over by CC and a well coordinated team. That's not a viable argument. And neither is the "learn to counter" argument. Why don't Darius players just "learn to counter" counters? Isn't that what ganking is for? Not being able to 1v1 doesn't mean you lost your lane, but not being able to outfarm/kill certainly does.
Darius just needs toned down a bit more, but I don't see him being toned down till they drop his IP cost. IP/RP cost seem to be the biggest excuse for a champion being able to remain "too good".