Split push, scaling, and opening the gates.
Empowered abilities and resource manipulation
Hitting an enemy champion with 3 attacks or separate abilities grants a burst of MS.
Shortly after using your Ultimate, gain a decaying Movement Speed increase that lasts for 3s and allows you to pass through units.
Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
Gain increasing amounts of AD or AP, Adaptive over the course of the game.
Durability and crowd control
Charge up a powerful attack against a tower while near it.
Restore health over time based on your missing health.
+6 Armor or 8 MR (match-up)
Non-damaging spells count toward proccing Phase rush so a simple [W]->[E]->Auto will let you hunt down your target and resist slows.
Nimbus Cloak's added mobility when casting [R] empowering you to chase down your target for repeated [Q] procs.
If you're playing into a number of AP threats, Nullifying Orb is also a great option to keep you alive in close fights.
More stacks ASAP. Nasus loves every CDR defensive item and with transcendence you don't have to worry about over-capping.
Usually when playing Nasus you're looking to come online in the mid to late game. Gathering Storm scales incredibly well along side you while you stack away.
Scorch is a solid alternative if you're looking to harass a bit in the laning phase.
Nasus is all about opening the gates. Demolish + [Q] means you're shredding towers in no-time.
Second Wind is a great sustain rune when playing into difficult match-ups. Like Doran's Shield, it allows you to mitigate a lot of the chip damage you might take.
In easy lanes you can take Revitalize to empower you late game [Q] healing.