Darius is an anti-bruiser bruiser. His range sucks and he is easily kited and has a hard time engaging vs any ranged champs. Unlike most bruisers he has no projectile to throw (nocturne, olaf, mundo), no slowing nuke (nasus), and no gapcloser(lots of people). This makes him terrible vs ranged harass.
HOWEVER, to compensate, he is extremely strong vs enemy melee champs with a short range non-melee axe swing harassment, high damage with his AxeSwing, autoattack, attackResetting Q, and bleed. His true damage ult counters the armor/MR stacking that many bruisers tend to do.
All this makes him a unique champ. Players that love melee bruisers all think hes OP as seen from that stupid response above.
Whereas the Ryze/Kennen type toplanes laugh at him and poke him to death.
The key to Darius in teamfights is SUSTAINED DAMAGE and TANKINESS. His axe CD is reaally fast and he can spam it during fights. His autoattacks hurt, and all his stuff builds up significant bonus damage from Bleed counters.
He also reallllly wants to be alive for when an enemy has 5 bleed counters and is near-death so he can Execute them.
The Stupid Darius Player builds tons of AD to get big numbers. He rushes in, gets off 1 or 2 Q,Ws and has to Ult before he dies when they only have 1 bleed counter.
The Smart Darius Player builds TANKY as heck, forcing enemies to expend tons of damage on him, or eat consistent damage from Axeswings and autoattacks, while Darius Executes them 1 by 1.
Darius mostly wants:
* A snare
* Magic Resist
* Cooldown reduction.
* Attack Damage.
* Crit/AS is nice but underutilized.
So you can see that good items for him include:
* Frozen Heart
* Frozen mallet
* Spirit Visage
* Aegis of Legion
* Guardian Angel
* Sunfire cape (so so, if health needed)
* Banshee (so-so, for health,MR)
* Atma's (good lategame for armor/AD)