I can understand some initial confusion at Blizzard's announcement that the wizard is not going to be using mana, but the fact that people are claiming they don't like the character as a result is simply baffling. Going back and reading the character description on the official site, the decision makes sense.
A couple of things that I took from the character description:
"When confronted by the great Vizjerei mage Valthek and demanded to account for herself, she brazenly attacked him rather than face the punishment merited by her acts. Exaggerated stories of the battle are already being inflated to mythic proportions by the more rebellious of our city's youth, but suffice it to say that she did not actually best Yshari's most powerful mage in single, honorable combat. The details of the encounter remain unclear, as Valthek has yet to regain consciousness, but it has been verified by reliable sources that she relied on trickery and deceit to bring the great man low. I have also been assured that the extensive property damage was chiefly the result of Valthek's magical prowess, not the upstart wizard's."
1) She overpowered a high ranking mage of her clan, at 19 years old. This report is written based on information provided by members of that clan, and is likely meant to reflect a conclusion reached by assuming someone so young could never have that power. The fact remains, however, that she won and it is very possible she did her share of that damage. We are dealing with a prodigy.
"...even the esteemed Vizjerei instructors were unable to rein her in. She was continually being caught seeking out dangerous and forbidden magics, heedless of the consequences to herself or anyone around her."
2) This character is power hungry and confident, a dangerous combination. She is sure that regardless of what has happened before, she can pull it off and do so with minimal consequences.
When I put this together I see a person who was born with innate abilities far above the normal person. Imagine a high end sorceress from D2. Eventually, you regenerated your mana pool so rapidly that it was no longer a concern. What would happen if you were capable of that at level 1? Mana would still be a resource you use, but certainly not a limiting factor you need to monitor.
So let's sidestep to the barbarian, who relies on fury. Basically this character is fueled through a blood lust. He runs off of pure adrenaline and uses it to massacre his enemies. Why couldn't this be applied to magic? Returning to the fact that this is an exceptional wizard dabbling in forbidden and dangerous magics with little regard for personal safety, it is likely she is willing to push the envelope. Imagine that, as the character uses mana with reckless abandon, there are residual effects. This can be whatever you want, but I like to think of it as magics being infused into the character's being. Instead of adrenaline, this character runs on an arcane infusion.
And there you have it, the new limiting factor is determined by the side effects of that infusion. We now know that this system will increase damage output while lowering stability. A lot of people like the idea, and some people just can't see it. Maybe this will help or maybe not, but I don't see what all of the fuss is about.