To be fair, it's David Walgrave himself who said during a twitch recording that the removal of mana was problematic as characters get more skills through the course of the game and thus cooldown are not an issue anymore.
I'm not a big fan of mana because it usually just cost golds and inventory in rpg with people having bazillions of mana pots in their bags. Or sometimes it costs "virtual time" (fight, sleep, fight, sleep, etc), so meh.
Honestly, I'm a big advocate of locked, limited skillbars during fights (and during fights only, I too want tons of quickbars the rest of the time). They would force us to make choices, and thus prepare some sort of tactics: Do I want high mobility for my warrior, with several rush and blink skills? or tons of Aoe? Or should he be a tanky beast with party buffs? Should my healing wizard favor single target, high heal spells? Or aoe, but less powerful heals? Burst heals or heal over time, etc.?
If we can access everything all the time, well all that decision taking disappear, we'll all make "tankninjamagehealers" and I fear it may make all the combats much less interesting than they could be, and turn into: Spam the big nukes first, spam the rest while they're on cooldown, repeat.
