I'm going to be honest here, I hate Darius. I hate his huge AD ratios, I hate that his ultimate gets an instant 20 second CD on a kill, I hate his mass pull that's not much shorter that Blitzcranks but hits everything, I hate the stacking bleed and the huge range AoE axe that does 50% more damage if you're on the latter half of its range, I hate the slow, I hate that he naturally gets 25% armor penetration, I hate that his ultimate does true damage that increases exponentially when you're inevitably bleeding from every time he hits you, I hate the CDR on his slow depending on how much you're bleeding, I hate his +100% AD bonus damage and slow to his next hit, I hate that he naturally gets fairly tanky early in the game, I hate that he has the lowest skill level in the entire game of LoL and I hate that if he kills you in lane you might as well not come back post 6 because he's going to kill you 3 more times and take the tower anyway.
THAT BEING SAID: I do think he needs a rework.
If Darius get's a kill or two in lane then he almost always snowballs into an unspeakable demon of unlimited ulti death, but if top lane starves him of a kill then he often ends up sub-par.
Darius needs to have his early game weakened and his late game reinforced. How?
Lower the AD ratio of his Q, Decimate (the AoE) from 70% to around 40% but keep the +50% bonus damage if hit by the head of the axe. Don't forget that applies a bleed of <30% your AD as well. This will promote an actual required use of skill with Darius to land the 50% bonus damage. Before you QQ that's still <120% your AD with the 50% bonus at level one with a 9 second CD. Any other champion who even goes near there does so with a skillshot not a wide AoE.
Crippling Strike (W): Increase the slow from 20 / 25 / 30 / 35 / 40% to 20 / 25 / 30 / 40 / 45% but decrease the bonus damage on his already insane AD ratios from 20 / 40 / 60 / 80 / 100% AD to 15 / 35 / 50 / 55 / 60% AD. Also change the CD from a flat 8 seconds (-1 for each stack of bleed on target) to 20 / 18 / 16 / 12 / 8 (-1 per stack of bleed on target). Why? This would weaken his early game while evening out his late game but simultaneously decreasing his likelihood to be kited because the extra slow would allow him a chance to kill the opponent first.
Apprehend (E): Keep the long range, but limit it to pulling the closest object only at level 1, then at each level beyond that you can pull one more object, maxing at 5 which is an entire enemy team and if you need to pull more than that into a melee slaughter machine then you just really suck at this game. This would increase the skill required to land pulls in lane, but shouldn't affect the late game unless you're really really awful.
Instead of giving him 5 / 10 / 15 / 20 / 25% armor penetration on his, again, MASSIVE AD ratios change it to 5 / 7 / 9 / 12 / 15% Armor pen and 3 / 5 / 7 / 10 / 13% tenacity (resistance to stuns, fears, slows, immobilizes, CC in general) This would improve his mobility in a teamfight and make him less of a total nightmare to fight against. 25% passive armor penetration? Really? Who the hell gets that on top of having every AD ratio in his kit reaching 100% or above. Who gets that at all, few do even with items.
Noxian Guillotine (R): Darius's "dunk", his signature move. This is his ultimate so it should be powerful, but balanced. Which it currently is not. So what should be done? Change the bonus AD% from 75% to 110%, but instead of dealing massive bonus true damage per stack of bleed Noxian Guillotine deals 80 PHYSICAL DAMAGE per stack of bleed and consumes the stacks. Also make the ultimate deal 80% of its damage as True damage and the last 20% as Physical damage. It would be mixed, kind of like Ahri's orb. The Cooldown would also change from being instantly on CD to a 4 second timer before going on CD for 10 seconds. This increases his ultimates power while also providing some counter play. This adds some level of strategy to playing with or against Darius.