Umm one item didn't fix mana problems. It just made it less of an issue, as in it came back just a tad faster than it did before which meant you could afford to run a full volley of spells (once, never more than once not just because of CDR but because you had to conserve mana to make sure that you could get away).. IE you finished Morello's and now you could cast 3 or 4 spells in a row without running out of mana or being critically low (depending on champ). Some champs could cast more than just 3 or 4 all in a row without pause (but those are more the utility style mages or Liss who gets some spells for free thanks to her passive). Before that you cast 1 spell, then waited till cdr and mana came back a tad, then cast again or you'd cast 3 in a row and then wait for the dorans to help rebuild mana and cd's to come up etc.. It was a waiting game always gauging just how much mana had come back and whether or not you had enough mana to run a volley of spells but trying to make sure you didn't run critically low so that when the enemy all-in'd you'd have enough to retaliate.
Generally speaking you needed 2 items but the regen meant that you could go a full volley without being critcally low. All mages are careful about their mana and are always careful about their mana. The entire reason they were "changing this" was to encourage mages to use their spells more often and not just use the lowest costing spell. Flat mana might help the early game with using more than the lowest spell or it might just make it worse. That does remain to be seen but in general mages aren't complaining that "buying one item" was the problem, the problem is an unreliable way of ensuring that your mana comes back. Without the item the mana regen is so very much lower and having an even larger pool makes Mages worry that mana will become a major issue when you need it most (team fights in the mid to late game). Previously, you still didn't spam your spells you used your lowest spell for waveclear and then saved as much as you could for a possible fight. If you didn't see anyone on the map and you wanted to back you'd throw as much at the wave to push it and then back.
Summary: Mages are worried that with larger mana pools it will take much longer to get to a high enough mana level to be able to operate in a team fight. Having lower cd's means more spells cast and higher mana pools means more ability to cast. However, low mana regen means that the high mana pool will take MUCH longer to refill at the base level mana regen for most mages. (For instance: Take any mage mid in a custom, spam spells till you run out of mana, watch how long it takes for the mana to regen on it's own. Many of us do this to see what our limits are for spell usage just to get a basic idea of how often we should cast each spell. Then do it again with Doran's ring.) Now imagine having a bigger mana pool to fill with that low of a regen rate.. This is what has mages worried. So having a higher mana pool and lower cd will not change mage behavior when it comes to spell casting in general because now they have the added worry of extremely low mana regen and higher pools to worry about filling up. Mages are worried that when it's critical without some sort of extra regen mechanic they will have to back before any team fights or objectives, not to buy but because the regen rate will be too low to sustain a higher mana pool and will make it unlikely they will be able to cast a full volley of spells to help in a team fight (as most mages are used for waveclear). Mages already have to back often to refill pools as it is (not all mages but again the ones with really high mana costs). As Mages are already critically aware of how long cd is for their spells and how much mana each spell takes the added problem of regen will make contesting a lot more difficult (more mana pools means longer time on the fountain to refill the mana enough to run the volley of spells). Mages already have to worry about mana pools, mana costs per spell and regen.. However previously there were options to help alleviate some of the worries and Mages are now worried that it will add more problems not less.
Irritations: Mages are fine with trade-offs for power, what they are not fine with is that every other class doesn't seem to have "gates" to their power and that Mage "power" doesn't seem to actually offer much in the way of power anymore. (Other classes "gates" seem to revolve around buying items and that's about it.)