Here's some math for all of you people saying that Mana-pools and Meditation will be insanely high:
Average Mana per Mage-Champions at level 18 is 1000--1300, with a few here and there that break 1400 and then Kassadin and Janna sitting at the top with 1500.
Will only give 400 Mana now after 10 minutes of stacking.
Gives 1000 at max stacks, so 7-10 minutes of stacking based on the champion using it.
Will give 400 the moment you upgrade it.
So all together the average Mage champion who will be buying these items just to afford to use their spells will reach a Mana-pool of around 2800--3100 at max-level with 3 full items, so exactly the same as right now since ROA gives us 800 Mana after 10 minutes.
So for those that are saying that Meditation is going to be buffed significantly by this change: If a Mage runs completely dry and maxes out the power of Meditation they'll reach up to a bonus of 42--46.5 Mana per five seconds at max-level with 3 full items, and that bonus will steadily decrease with every % of Mana the Mage gets back. And guess what? With no other Mana-Regen stats in your entire build that equals dick all in terms of Mana.
So I'll say it again: once a Mage inevitably runs out of Mana, since the only way for them to push and clear waves is to use their spells, they're screwed because they have no way to get it back other than tanking enemy damage.

, those that scale with it
or those that need gigantical mana pools