The Q CD reduction on Siphoning Strike during Fury of the Sands is too strong
Mostly during his mid game, it's pretty disgusting because Nasus usually has his 40-45% CDR at this point. I am biased but I don't think it's an issue late game, it's not even truly an issue during the mid game in terms of reducing his weaknesses. They are still there but a few things are making my dawg disgusting right now. Here's the build I've had the most success with and it's ridic:

scaling MR blues Scaling HP yellows Flat AD reds Armor quints
0/18/12 Stormraiders Surge.
This build has basically deal with my weaknesses the best while offering some nice utility. Righteous Glory is ridiculous - this isn't Nasus fault, the item needs nerfed. Thornmail is interchangeable with Randuins depending on the enemy team comp. I truthfully prefer trinity over IBG but because RG is busted and IBG is about 900 gold cheaper, I get a really big spike early with more slot efficiency. We all know the synergy with spirit visage, its one of his bread and butter items.
I love the Q reduction on R but it's making his mid game spike too hard. One of the rioters proposed a change from 0/25/50% from levels 6/11/16 and I think that's a good place to start. In exchange, make nasus' max hp% burn on ult 5% at all levels up from 3/4/5 to give his early/mid game a tad bit more power so he can compete in this hyper accelerated pace metagame. He's just currently scaling too hard and too fast.