Karthus is an incredibly mana hungry, immobile mage. His Q has a long delay on it and deals significantly less damage if he hits multiple targets. If his Q drew aggro, enemies could wayyyy too easily deny him farm by walking into it, halving the damage to minions and causing him to take minion damage. If you are being out farmed and zoned by Karthus, don't farm out away from minions where he can deal high damage to you.
He's got loads of weaknesses, so I would hardly call him out of line, and when he didn't have the buffs he got this season he was completely invalid. He's in a decent place right now, so I don't think he needs any kind of nerfing.