As Righteous fury does splash damage and has relatively high Damage output boosting Attack Speed enables you to pump out massive damage. Especially when this is coupled with Reckoning and Holy-Fevour this means you can be pushing out HEAPS of damage per second. The Cooldowns on the Glyphs aswell as the Items enables you to keep your Righteous Fury up nearly Permanently and pump out your Reckoning, keeping your enemies slowed and susceptible to your power.
The Mana-regen gives you sustain, but is interchangeable.

Outside of picking up improved Flash, Exhaust and picking up magic penetration (and cooldowns) these are very interchangeable, but most of the focus has been placed on pumping out massive damage, I might recommend subbing something out for improved Move-speed because Kayle is ridiculously slow as a champion

Similar to the runes description, pumping out massive amounts of attacks to improve usefulness of Righteous Fury. With the added cooldowns on Nashor's you can usually keep RF up permanently and the increased AP just allows an increase in Damage.

For Slowing and Maneuverability... What else did you think they were for? Idiot.