Everything about jungle Nasus is boss now!
Armor shred on spirit fire combos so well with LW and BC that the AD carry deals true damage. Maxing spirit fire first also combos with new potion bag as a quick purchase. (I like to start with dorans and buy the potion bag next).
Ult counters health stacking which is the main way to tank now that armor/mr are more expensive.
Wither counters AD carries now that armor/mr are more expensive. It also lets you gank and counter-gank non-stop regardless of buffs and jungle changes (especially combined with potion bag). I max this 2nd.
Base lifesteal counters lack of lifesteal options, gives sustain for the new jungle, and allows you to start with boots, dorans blade, or machete.