The key here is being able to spam W & E without sacrificing Q farming. As you advanced from level 1 to level 5 in Siphon the sustained mana cost QUADRUPLED because cool-down was halved and cost doubled. At level 1 you were using 2.5 mana per second which grew to 10 mana per second if (2.5, 3.1, 5, 7, 10) if your mashing Q the instant it's cool-down finished (which you had better be if you expect to be the beast your meant to be).
Now that only grows to 5 mana per second (2.5, 2.8, 3.3, 4, 5) the difference of 5 mana per second or 25 mana per 5 seconds is huge, it's the equivalent of adding 8 Faerie Charms to Nasus, and is almost enough mana to cover the full cost of spamming level 5 Wither every single time it refreshes (total cost would be 7.2 mana per second), or covering half the cost of spamming lvl 5 Spirit Fire every time it's refreshed (10.8 mana per second). You will definitely notice this by level 3 Siphon Strike, which you usually have around level 5, and by level ~8 when you max Siphon strike the benefits will be big.
Imagine being able to constantly Wither your opponent in lane as a sustained denial of his CS, rather then just as an attack/escape mechanism. I like to give em an occasional smack with Siphon Strike too while Withered, this gradually forces them out of lane as you stay so topped off in health they can't risk engaging you and any harassment they try just falls off.
With this change Nasus should be able to dispense entirely with mana regen items while farming without CDR and with CDR and a small amount of mana regen he can farm at a frightening rate, using spirit-fire to blow through minion waves.
The CDR buff change to Spirit Visage balances the nerf to Glacial Shroud so not too bad. My build was usually to go for a quick Vamp Scepter for infinite no-potion sustain then Glacial for CDR and then Glacial fist for slows. Now I'm probably going to be going Spirit Visage before Glacial fist at least vs anyone who has magic damage.
I also might stop carrying Teleport, I found many of my uses of Teleport are for mana when laneing, that shouldn't be an issue now so
Thanks. I guess that was the most cohesive explanation.
But Hyfe did justifiably mention that it does nothing for his first few levels. Which isn't too bad.