Well I only play Dominion, so I'm not sure how this translates to SR, but Darius:
1. Is extremely vulnerable to kiting. Bait the pull and point and laugh the next 15 or so seconds, since he can't do anything unless you smush your face against his pecs.
2. Starves his team of kills something fierce. I've started to love it when the enemy Dariuses get ult-happy because there are other characters which benefit from kills, while Darius likes to build tanky and deal damage through his passive and ult, and if he goes carry build he explodes. Yes, it does a lot of damage even without damaging items.
3. Has no way to initiate apart from pulling, so if he goes berserk he explodes.
4. Is vulnerable to ganks, especially if he overextends, since he has no escapes.
5. Has no built-in sustain, so you can outsustain him.
I might be talking out of my arse when it comes to SR, though. Darius was extremely scary bot at release since noone knew what he did and how exactly he did it. Now whenever I see him bot I just kind of feel sorry for his team. What I've heard works reliably is counterpicking him with Kayle, possibly because she can kite extremely well and fuck with his ult via her own ult. I suspect Tryndamere would also work, but I haven't heard anything about tryn recently, so I'm not sure.
Most talk is Darius being OP is on SR, Dominion is a completely different game in regards to the power of champs especially with it's much more open playing space it is easier to kite champs, in SR though there are tons of obstacles including your own minions at times which makes it much more harder to kite. Even then your points are weird but I don't play Dominion so really can only argue on SR.
1. Is about as vulnerable to kiting as any melee champ, yes if he misses the pull you can kite him safely but he has a pretty good range on his Q to still be able to control a lane early game. Just because you can kite a melee champ doesn't mean they just always try and chase you, early game Darius can control the lane and he does huge damage just with his Q and passive to kill very easily early game if you try and get some creep kills as well. Just being hit by his Q can give him enough speed to chase you so you have to stay far away from him.
2. It doesn't really matter if he is starving kills of other players, it's saying the same of Akali or Katarina who also get cd reset/reduction on kills/assists and deal shit loads of burst damage. He can get penta in teamfights so towers get pushed or gets to kill dragon and/or baron which is more then enough gold to offset "starving" of other champs.
3. His Q initiates if it lands as well by giving him a speed boost, super effective if hitting more then 1 champ, easily enough to give a speed boost to catch up for a pull, and he can do a nice big slow if he gets into melee range as well.
4. He does have a harder time running away although he does still get a nice speed boost for bleeding 2 champs but the issue with Darius is that even though he does shit loads of damge without the ulti, with his ulti it is pretty much an "enemy has health <50%, EXECUTION!" situation. You need to kill him completely, he just needs to get you down the half health, the fact that he deals true damage means only noobs Darius will fail to execute as it's easy to know when exactly it will kill an opponent.
5. No sustain? Because every other champion starts with spell vamp and lifesteal as passive? He doesn't need to sustain because he will bleed you faster then you can bleed him and then he will kill you laughing at your pathetic "sustain".
Kayle, Tryn and a few others can troll his ulti but even without the ulti he can deal a lot of damage in a short time. Getting 5 stacks of haemorrhage is like a constant ignite, his ulti is not neccessary for him to kill which is why he can get an easy first blood if an enemy champ is stupid enough to try and out-sustain him.