Darius Main here. Hi.
1, his passive has BEEN nerfed. It used to chunk massively before his rework, so the passive you see currently is the weakened version. What you're frustrated at is his 5 stacks AD buff, which adds onto the damage for his passive (since there is an ad ratio on it)
2.Darius is extremely strong close up, yes. He's a juggernaut. That's his role. When interviewing riot, the developers themselves said that they liked the idea of "almost guarenteeing losing a lane", since he's so dominating. His weakness is the fact that he can't get to anyone, and thats a massive weakness in this high mobility/burst meta. So many champs have strong disengage/cc to disrupt darius that its insanely difficult to get on them. I could take ghost, yes, but that cuts down on my map presence, and ghost doesn't give me tenacity.
- It sucks to fight him for 2 reasons. You either can't dodge his heal (I'm not critiquing your skill here: Some champions are physically incapable of dodging his Q due to champion sizes and hitboxes), and his passive ad burst. However, that means Darius has to get to 5 stacks, and land the outer ring of his Q. If you have a dash, you can quite literally avoid both threats here. Furthermore, you can use the dash to go into the darius, so he can't heal/get a stack of passive, and is stuck in an animation while you are free to wreck havoc. Furthermore, even though he has an auto attack reset, he has one of, if not THE most, lowest base attack speed in the game. As such, you can have auto duels, then use a dash to disengage when at 4 stacks.
TLDR: Darius is strong when you play around his strengths. So don't fight him in his strengths. Fight him in his weaknesses. Higher elo can capaitalize on this, and thats why juggernauts tend to steamroll lower elos, and suffer higher elos. It's worth it to be 40 cs behind early game. Let him take your turret if your jungler won't come top, then farm safely midgame. Later on, in 5v5s, if your team decides to focus him, he can't do anything. A lowrange hook is all he has, and in this meta of mobility and burst, missing that hook means death.