Permaslow? It's pretty meh at early ranks, and it's easy to dodge.
His Q is his gap closer, but it's on a significant cooldown until fully ranked. If he uses it to engage he can't use it to escape, and Aatrox caves under heavy pressure.
His passive isn't broken. If he can die, survive for the five minute cooldown as relatively squishy as he is if he builds damage, he can get another passive off and I don't mind.
The knockup? It only applies to the center of his landing radius, and isn't even that big of a knockup. It's pretty quickly recovered from, and altogether avoided with any movement ability or, say, Flash.
His ult is honestly the only thing that gives him any ability in teamfights. Most Aatroxes are either too offensive to stay alive against multiple enemies, or too defensive to deal enough damage to matter. His ult allows him to ramp up his meager offtank damage, and give him some more sustain if he's building straight AD.
He isn't broken, not even close.