I have a few specific issues.
No strategic counter spells/spell defense/dispel magic-
This relates mainly to a few of the strategic spells, such as Curgen's volcano and grip of winter, but also touches on many of the buffs/debufs.
Spells like Curgen's Volcano are very nice, very powerful spells (There should be more of spells like this, next issue), good job. However, the concern I have is that there is no counter to these spells. Wow, I just got access to Grip of Winter and have been storing up mana since turn one... now what? Looks like I can cast the I win spell, and I'm not talking about the spell of making. Now you can't train, your economy has just been halved, and there's nothing you can do about it. Even if you've also been saving up mana since turn 1. Same with Curgen's Volcano, looks like an I win button unless there is some sort of counter/defense. If you include super powerful worldwide enchantments or supremely powerful strategic damage spell, and these definitely should be included, you also have to include defensive counters to these spells. Anti-Magic bubbles to guard cities. Dispel magic to remove world enchantments (and those anti-magic bubbles so my offensive spells are useful again). Allow me to use my mana to both destroy my enemies or to protect my own territory from my enemies.
Not enough powerful world enchantments/powerful strategic spells-
You basically have three. Curgen's Volcano, earthquake, and Grip of winter. You need more. Every school should have big global enchantments like Grip of Winter. Look at AoW. Each school had its own global enchantments that each had profound and interesting effects on the game. FE still has very little of this.
No mana cap-
Huge issue here. Basically, because mana will be able to stored without limit, the most powerful magic using empire late game will be the empire that never uses magic prior to the late game. Seems a bit counter intuitive to me. While there should be some trade off between storing mana and using mana, the system as it exists seems to go to far, and will discourage any use of mana in the early game for fear of not having enough when the volcano's start erupting. If I want to have huge mana reserves late game make me focus on magic throughout the game, building mage towers, mana banks, etc.
Having a simple mechanic like a mana cap would go a long way to removing this problem. There is still an interesting tradeoff between using mana for big strategic spells and smaller tactical spells, and even a tradeoff in terms of when you use your mana and saving vs. spending. But it also grants the freedom to cast fireballs on turn 10-100 without making me worry that I'll be crippling myself at turn 500.
Add to that the fact that you can then include buildings that increase the mana cap and you have another interesting tradeoff. Do I construct buildings that increase my mana cap so I can focus on magic, or buildings that increase my economy or military production?
Without something like this I think its going to be even harder to balance the big powerful spells that we all want to see included. Volcano's are great, but if I can destroy every single one of my enemy's cities with no warning the turn after I have access to them then there might be a potential balance problem. With a mana cap, I can destroy an important city, but I because of the cap, there's just no way I can do it again in the near future (unless I take some penalty like 'Sacrifice', which is a great spell).
Lack of balance between spell schools-
There are a few spells that will have a disproportionate impact on the game. These spells will, in my opinion be very difficult to balance leading to greater problems down the road. A few obvious examples:
Cloud walk: Teleport by another name. Completely changes the concept of strategic movement within the game. A player with Cloud Walk is playing a different game than a player without Cloud Walk, making Air almost a mandatory choice. Tone this down (make it only the caster, scale mana cost to army size, etc.) or just remove it. There's no way to balance something that so drastically changes the way a fundamental mechanic (movement) works.
Mantle of Oceans: Another one that affects a fundamental mechanic (the mana economy) in a rather drastic way. Someone with one water IV mage can now cast twice the number of tactical spells as someone without a water IV mage. Why is this effect in the water school anyway? Contrast with Mantle of fire. One does 4 damage to a units that attack me, one halves the cost of ALL tactical spells. Both are level 4. One of these things is not like the other.
Curgen's Volcano and Earthquake- Boom! population halved, city destroyed. As I said before, with some sort of defensive magic system, this problem is somewhat mitigated, a civ specializing in life magic would at least have some defense. However, if a defensive magic system is not implemented these spells are WAY overpowered. One line of magic, earth can halve an enemy's population and with it their economy. Unless cities are going to be drastically reduced in importance these seem like overpowered spells compared to the other level 5 strategic spells.
More summoning spells-
Echoing many others here. You guys can do better than a few bland elemental summons. Come on, these are boring!
Lack of an Arcane/Sorcery/Metamagic school-
This has already been mentioned, but I second it.
The system needs a generic magic category for many of the effects that just don't fit into any of the schools. See my next comment about water magic for the worst offender.
Make the school available to a wizard with the requisite rank in any of the disciplines. This would enable you to further specialize the schools while still retaining some basic spells for all magic users to access. E.g. a basic direct damage spell, dispel magic, a nerfed healing spell, a few low levels summons, some low level buffs/debufs, etc. Then the specialized schools give the really powerful versions.
Water Magic seems like a collection of random effects-
Small issue, and one that's probably not really solvable until there is a major overhaul to the way the sea is used in Elemental. Other than the ice based spells, most of the stuff in the water school doesn't really have anything to do with water.