Basically, Darius has a very defined role. He's an early game bully, and if he has a strong early game because of it he's a mid game nightmare. However, because his strengths are so straightforwards, he also lacks in other areas. He has minimal CC, no gap closer, and no mobility options. He's very easy to do well with, but he's just as easy to not do well with. If Darius falls behind, you feel it. He HAS to be ahead to be dealing the kind of damage that makes people fear him.
If you buff him, you run the risk of making his early game more monstrous, or making his late game stronger than it should be given how powerful his early and mid game can be. If you nerf him you run the risk of making his late game even more useless by having him fall off harder, or taking away his early game which is where he has to shine to do well in a match.
I'm not saying that Darius is a bad champion, not at all. But he has very clear strengths and weaknesses, and he fits into compositions that can capitalize on that well. He isn't really a safe pick for solo queue, because he can need some heavy help from teammates going into the mid and late game to set him up to really do well and allow him to unload in teamfights, but with a team that can support him he's amazing. He's very much like Renekton, who's strength is a sustained sort of bullying pattern through the early and mid game, someone who doesn't have to leave their lane and who constantly pulls the enemy jungler to focus more on the top lane, letting laners in other areas push harder. He isn't someone you take when you're trying to play it safe, he isn't someone you take if you're trying to be a disruptive tank, he isn't someone you take if you're trying to supplement a team that needs some utility. His entire gameplay is balanced around the fact that his job is to be a bully against single targets but fall short against multiple enemies unless he's HEAVILY fed, and to that end he does quite well.
If you feel that he needs buffs or nerfs, before posting in the boards, I would suggest you take a step back and analyze yourself first. What were you doing that made it seem like you were weak when playing him? What was he doing that made him seem strong when you were playing against him? What sort of behaviors and builds do you see being used by Dariuses who are ahead, even, or behind. You'll start to see why nudging him one way or the other can quickly spiral out of control.