On paper, he seems like he'd be a monster right now. He has insanely high base damage and deals mixed damage, which makes it difficult to itemize against him in lane. He gets a significant amount of free armor penetration and true damage to deal with armor stacking tanks like Mundo and Shyvana. His Q is on a very low cooldown, which allows him to be a huge threat in teamfights while building like a supertank (Sunfire, SV, Randuins, Thornmail, and triforce or hydra).

Furthermore, the S4 changes really helped him out. Free armor penetration through masteries is much more difficult to get now, effectively increasing the value of his E passive. Warding trinkets give him more vision, making him less susceptible to ganks. Talisman is extremely powerful right now and most supports are picking it up - this gives him mobility in teamfights and makes up for his lack of engage, which are his biggest weaknesses. He also synergizes well with Sivir, a very common pick. Finally, his Q gives him decent waveclear and pushing potential, which is useful in a splitpush scenario and decent for holding 2v1 towers in competitive play. Why do we not see more Darius?