It may not help, but here's my thoughts on the stuff you talked about.
The passive is 200 bonus ad at level 18. It's 40 bonus ad at level 1 and scales up at about 9.5 ad per level. I don't think that's the unreasonable part when it comes to playing against him. The passive is also completely meaningless without getting the 5 stacks. It's not like he gains 20% of the bonus AD per stack; you actually have to work to get every single stack.
I'll agree getting stacks in a 1v1 scenario isn't too bad, it can actually be quite easy if you combo correctly, but it's much harder in full 5v5s. Here's why: if Darius wants to proc his passive really badly, he will continue to attack the same target, regardless of who it is. Doesn't matter if it's a Maokai or a Xerath since he can't carry over stacks on other targets like Jax passive. So by forcing him to attack that single target because he wants the passive proc, he's not focusing other targets. It works fine he gets to the ADC or mid, but not so much when he's just hitting the tanks because he can't get to the ADC. Sure he may get the proc on the tank, but the ADC has been hitting him the whole time and now he's less than 1/4. Now, he may not be able to make use of the proc other than just to finish a tank and hoping he can get a heal from his Q.
In regards to a "powerful true damage ult," Darius ult deals 100/200/300 true damage + 20% per stack of bleed + 75% bonus AD. For comparison, Cho'Gath ult deals 300/475/650 + 70% AP. So, without passive stacks, level 16 Darius ult deals less damage than level 11 Cho'Gath ult, almost less than level 6 Cho'Gath ult. Even at max bleed stacks, Cho'Gath ult would almost certainly still outdamage Darius ult at max level, abut 750 (assuming we only take in the bonus AD from passive, not items) versus the base on Cho'Gath being 650. So Cho'Gath only needs like 180 or so AP to outdamage Darius ult. So in a sense, Darius has to work for an effective ult while Cho'Gath doesn't.
On the same point, his ult is easy to mess up. That sounds funny, but I can't tell you the number of times I've ulted and guessed wrong by 100 hp or so. That, to me, is the most infuriating part of playing Darius. Lucky, I sometimes proc the passive and the bleed finishes it for me, so I can type "calculated" and feel better.
Part of why the ult is easy to mess up is because of the passive proc. The proc honestly gives a feeling of something like, "it's to f*** some people up." This causes people to just ult the second they get the proc and, more often than not, you fail because you didn't get them low enough to actually execute. Especially in teamfights when you do get a reset, if you just blindly ult without applying passive or looking at hp's so you're sure you're getting another reset, it's not going to be enough and your ult is now on CD. In respect to the passive's bonus AD, if you can't get the stacks like I was saying above, then your only hope is a reset with an ult kill. Ult damage is marginal without the full 5 from passive, so you almost end up in competition with your team to get a reset with ult unless the team intentionally donates to you so you get the reset. Many kills have been lost mid ult animation, many more because the reset wasnt triggered. Plus, his reset mechanic is unique in how he must get the kill with that specific ability, whereas Kha'zix, Tristana, and Katarina get resets just by being involved period. Obviously his is an ultimate that deals true damage and just got a bunch of bonus AD for the execute and their are mobility for Trist and Kha'zix and CD for Katarina, but the philosophy on the reset is unique for Darius vs. any other champion.
On the subject of the % armor pen: this one is definitely tricky because flat % armor pen doesn't really exist anymore since LW is now % bonus. This does create really really nice synergy with Darius. A few things though:
I. Cleaver almost must be stacked to be meaningful. It goes up to 30% reduced sure, but it takes 6 stacks to get there. In a 1v1, it's no big deal; but just like Darius passive, it's a little harder to distribute the shred, even though that's what Cleaver is about. Darius wants to selfish with passive stacks, but needs to distribute Cleaver stacks to help the team cut through tanks. See the problem?
II. Cleaver and the E are nice synergy for the rest of his kit, but do not synergize at all with his ult. Since his ult deals true damage, armor pen and armor shred are meaningless numbers and have no effect on its damage. That may seem obvious, but actually aknowledging that shows an interesting disconnect in how the pen and shred helps his Q, W, and basic attacks, but not the most recognizable part of his kit.
III. I may be wrong about this, Darius E armor pen is basically ignoring a person's armor, while cleaver is negating armor entirely from the champion, hence the term "reduces armor" vs. armor penetration. Darius doesn't ignore 55% of a target's armor; rather, he attacks once, basic pen % applied, the the shred happens. The pen is reduced in effectiveness based on total armor as more armor is shred. In a sense, you're negating less armor with the E's % pen because less armor is on the champion after the shred is on. The difference between the two methods of pen and shred (the 55% vs the normal method we have now) is about 10% total armor preserved with latter method. Having that synergy in front of you makes it seem less dumb. At least that's how it is for me.
The synergy is still nice, don't get me wrong, but it sounds less stupid, imo, when you look at it like this. It makes a little more sense to keep the pen where it is when you think through the flaws it has.
Personally, if I knew Darius had to be nerfed, I'd think one of these 3 things would be appropriate:
I. Decimate (Q) CD nerf
II. Crippling Strike (W) mana refund on kill lowered OR CD reduction removed
III. Crippling Strike (W) slow nerf
Tl;Dr I don't think Darius gets too many stats "for free", per say. I think he has to actually try and get the bonus AD from passive and it forces decision making in teamfights based on conflict with Cleaver's shred. The ultimate is easy to mess up on and harder to get resets in teamfights late game. As far as true damage ults go, he has to work for good damage while Cho'Gath doesn't. E and Cleaver synergy is nice, but doesn't help his ult and Cleaver stacks conflict with his passive stacks to a degree. 3 nerfs that I'd consider would be Q CD, W slow, and W mana refund or CD refund.