He's not overpowered, he's toxic.
There's a difference. When I'm playing soloqueue I drill it into my top laners head not to let Nasus farm. Like when you're against a Nasus, that's your only task.
If you keep losing to a Nasus, then you need to sit down with a notebook and write "Do not let Nasus farm." over and over again until you get it. Seriously though, it's so frustrating when you're doing well, but you realize that Nasus has been free farming because your teammate has not been bullying him out of lane and freezing the wave. Most champions do pretty well against Nasus early on.
A tank doing massive amounts of damage is absurd, but it takes Nasus so long to get there if you monitor him. There are other tanks who get there much quicker now. A Nasus who farms isn't the most skilled player ever, and they don't deserve high praise like some people here have mentioned. It's more like your top laner screwed up and let him get to where he is uninterrupted. That's why it's toxic, but not overpowered.