I didn't read all of that, but I'm going to suggest a system and let you critizise it :)
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how's the basic spell going to be cast, the wizard can't just whack someone on the head with a stick. 0-cost spells or something?
There is a default regeneration, like with mana, that makes sure you never start a battle without any energy. It would work like the grace period of Fury, but reversed. You regenerate when you are outside of combat (not dealing damage) and stops regenerating when you attack.
There is no "basic" spell as it changes with your build and skill selection. A skill might cost you energy in level 1, but give you energy in level 5 with a certain rune. You can choose to play balanced and use a basic spell that deals moderate damage but drains/gives very little energy. Or you could use one weak spell with a high energy gain, together with a high power/high draining spell. You could focus your build on a large energy pool, or a higher energy regeneration, or a better energy gain, or more damage, or whatever.
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A barb suffers from not fighting, which counterbalances the fast increase of fury from fighting. Wizard? They can easily keep that supply of mana high if they just keep casting free spells, and free-spell-spam is not uncommon in games that permit it.
Correct. You can continously cast completely free spells, but that is no different from many low cost spells in a mana system.
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They usually can't be completely useless since you need to let the wizard recharge somehow. I could see already them becoming the main spell a wizard uses all the time. I could also see it getting boring really fast. Cast ray of frost 10x times then cast blizzard then cast ray of frost 10x times than cast blizzard again and so on.
Imaginary example of Barbarian gameplay:
1. You use the skill that gives a little fury boost and charge into battle.
2. You use some low fury cost skills to build up fury.
3. You use your most powerful AOE skill or your most powerful single target spell. You will regenerate fury for as long as you do enough damage.
Not much more to that, pretty repetitive. You start out weak and quickly build up to a continous high damage output.
Witch Doctor gameplay:
1. You summon your minions.
2. You cast support spells.
3. You start spamming damage spells.
Also pretty repetitive. Your damage stays stable over the whole fight as long as your mana is high.
Wizard gameplay:
1. You start by using your most devastating spells.
2. You use weaker spells that regenerates mana to build it up again, or more stable spells.
3. You use devastating spells again or stay with the more balanced spells.
Also repetitive. You start with very high damage and switch to either stable spells or more varied damage output. But there is one difference from the Barbarian/WD: The Wizard must run because it has no tanking capabilities. This opens up the field for slowing and debuffing spells like the slowing bubble. It might not be able to just stay and do the repetitive weak, weak, weak, strong spellcasting. It might have to replace some of those weak and strong spells with spells that aren't intended for damage.
The ideal fight would start out with some spells that drop the energy to around 50/100. You would then use varied single target/AOE spells to stay at around 50/100. A bit like the Monk; in cycles of 3 spells in a combo.
I think this would work fine and be pretty varied. You can't just use spells and estimate how long it is going to take until your mana pool is empty, and then you are screwed. You have to come up with good combinations of spells that keeps your mana balanced. You can't just go with 100% devastating power spells without having some less powerful spells for your left mouse button.