Counter picking is great and all, but what happens if the other team has last pick? Darius has a strong advantage against most top lane champions, though there are a few that turn the tables on him. Should you pick out of that latter subset? Or do you risk being dominated by Darius because his strength is so phenomenal when he isn't being countered?
The problem isn't that Darius is op; he isn't.
The problem is that the majority of the counter-play to Darius happens in champion select. Either you pick a champion that beats him, or you don't. Late game, since his strengths are harder to apply, you may still win the game...but there's still the soul-crushing experience of being absolutely destroyed in lane with little to no chance of doing anything about it.
TL;DR LoL is fun due to the play/counter-play interactions. Darius is offensive because his counterplay is mostly in champ select. He is not, however, op.