Mana regeneration is now non-linear, allowing us to have most of the benefits of having mana that you can run low on (or out of entirely), but eliminating the worst problem with levels where you have no access to other attacks or to recharging items - the vicious cycle of using a puny attack, running out of mana, using another puny attack ad nauseum.  This in principle makes attacks scale in cost depending on how much mana you have - attacks with a high cost remain expensive at all times, but attacks with a low cost effectively become considerably cheaper during situations where they're desperately needed.