"just sit on him if he uses his Q"
Should have been more specific my bad, Darius does always wins the war of attrition early. Yeah if the more mobile duelists are smart going in for their short burst damage trades after baiting his Q is how they win in the lane.
However duelists that are hardier like Olaf with Black Cleaver or Tryndamere and Jax with BoTRK and their Ult's can just run down Darius and kill him easily.
Have you ever seen Hashinshin stream perhaps? He counterpicks Olaf with Ghost into Darius(and Garen btw) all the time. During the laning phase once he gets Black Cleaver he just fishes for an axe, pops his ult and maybe his ghost if he needs to turret dive and just kills the Darius for free. Like, every time, free kill, once he has ult back up.
Also I know this is somewhat "in the laning phase discusion" but this movespeed thing will apply to all stages in the game. In the late game don't underestimate the damage output of some of the mobile duelists like Yasuo, Riven and even Fizz, have especially once they have lifesteal items. They will chase down and tear down even true tanks easily 1v1. An immobile off tank like Darius? He will die, he will of course do more damage to them than a regular tank would, but he will die, without contest.
As for the landing constant Q's on some opponent's without any retaliation, well that could be many factors.
They are unfamiliar with the skill, which is something that everyone needs to relearn now, both the people that play as and against Darius.
Some champions can't avoid(or take advantage of) the skill easily as others, they are at a disadvantage, and may have been counterpicked. Some actually think of the team rather than their match up and pick champions based on that and that leaves them at a disadvantage. Or like most people right now downright refuse to pick smartly against some of the new champions and counterpick themselves. These things are ok to have in a game like this.
Decimate may be on strong right now and is being looked at, the heal being reworked and the mana cost per rank will increase. So it wont be so spamable as much at lvl 9(when Q is at max) and the heal will be less impactfull in 1v1 or 1v2. This should help the melee match ups hopefully. (Its actually better in teamfights now that the heal cap is raised, which is better for the vision of Darius they have now.)
The skill is punishing to some melee champions yes, it is meant to be that way, getting close to him carelessly is BAD. It has more counterplay than its old form so there's that. His laning is many time's healthier than before from all over his kit towards melee champions. The Magic Damage Bleed is gone, so is the W attack speed debuff, and the E bug fix means he is less sticky and no more wasted flashes.
I'm sorry but putting a slow on the only way he trades well in lane to nerf his lane when he already suffers from mobility issues early is just sounds like a horrible idea to me. The Q fixes hopefully are enough for people to stop worrying about his already weaker laning phase.