After you have all tank items, the enemy will laugh at how your Q's still only do ~200 damage after all their armor is factored in. Nasus needs to be built tanky, but he also needs to be a threat in fights.
Frozen Heart is core on Nasus because of the CDR, Armor, and aura. TriForce is also considered by some to be core, but I think that it can be delayed (heavily at times) if you need other items first (although Sheen and Phage are awesome). Last Whisper and Bloodthirster are both endgame situational items because a tanky Nasus with a farmed Q can gain back tons of health. Guardian Angel is always a good item to consider at any point in the game. Atmog's works well on him. Based on the bracket and game progression, Thornmail could be considered. A guide I read said to follow a progression like this: Merc (Tabi), Glacial Shroud, TriForce, FH, GA, Mog, Atma's.
His main threat is the ability to Q spam targets, so if you don't supplement your damage at all, then they will build defense faster than you can farm your Q and it will deal less meaningful damage as the game progresses, to an extent.