I'd like to know!

The options are the 3 most complained about things about Nasus.

1. People can just walk out of it, slow cast time

2. base health, health regen, mana regen etc.,

3. Pertains to the Q.

the Infinite Q

Definitely the Q.

It's worth noting that his W is the best slow in the game as well.

He has low mobility.

As much as I hate to say it, his infinitely stacking Q.

Because it is potentially infinitely powerful, the rest of his kit has to be scaled back so that he has to have a miracle match up to farm it. Give it a cap, and allocate the sacrificed power to the rest of his kit - perhaps even boost his range, a little bit?

I like to farm for 40 minutes then automatically win the game, Riot please don't make Nasus a strong and competitive champ throughout the entire match.

there is something holding Nasus back, this is news to me.

The potential for infinite scaling means that he needs to be infinitely terrible at some earlier phase in the game.

Cap his Q.

Then he can be balanced.