Improved Counterspell
Description:
Gives your counterspell a chance to silence the target for four seconds.
Ranks:
Rank 1- 50% chance to silence the target
Rank 2- 100% chance to silence the target
Usefulness:

Commentary:
Another talent that is best used in PVP. Not only will you be locking down an enemy's school of magic for eight seconds, but you'll prevent them from falling back on their other schools for four seconds. It is also good for when you mistime your spell and the enemy finishes their cast. Or, if you really need to silence and enemy to prevent them from using instant cast abilities. There are also some applications for this in PVE when fighting casters solo or in instances, but you'll most likely want this for PVP.

Arcane Meditation
Description:
Allows a percentage of your mana regeneration to continue while casting.
Ranks:
Rank 1- 10% mana regeneration
Rank 2- 20% mana regeneration
Rank 3- 30% mana regeneration
Usefulness:

Commentary:
Another huge boost for your mana efficiency. It's pretty much a requirement for most arcane builds since they suck up so much mana and most raiding builds because you have to fight for so long. How effective it will be for you will ultimately depend on how much spirit your gear has. But even if you have hardly any you still might want to pick this up and use mage armor.

Improved Blink
Description:
Increases the chance that all attacks will miss you by 25% for a few seconds..
Ranks:
Rank 1- 2 second duration.
Rank 2- 4 second duration.
Usefulness:

Commentary:
This is a much better improvement over the old version of improved blink.. It makes it a much better escape spell, though if you have blinked out of range then you shouldn't be taking much damage for a second or two anyway. You'll only ever want to take it for PVP builds though since it has very little use in PVE.

Presence of Mind
Description:
When activated, your next mage spell with a casting time less than 10 seconds becomes an instant cast spell.
Ranks:
One rank
Usefulness:

Commentary:
This is an incredible ability. The uses for it are practically endless, especially in pvp. Its most popular use is with pyroblast for huge burst damage. But it is also useful for an instant cast polymorph if you are attacked or ice bolt to slow an enemy.

Arcane Mind
Description:
Increases your total intellect by a percentage
Ranks:
Rank 1- Increases intellect by 3%
Rank 2- Increases intellect by 6%
Rank 3- Increases intellect by 9%
Rank 4- Increases intellect by 12%
Rank 5- Increases intellect by 15%
Usefulness:

Commentary:
There is no such thing as too much intellect. This not only lets you cast spells longer, but gives you a buff to your spell crit and will boost your DPS if you take the talent mind mastery.

Prismatic Cloak
Description:
Reduces all damage taken by an amount
Ranks:
Rank 1- Reduces all damage taken by 2%
Rank 2- Reduces all damage taken by 4%
Usefulness:

Commentary:
About the only time when it is worthwhile to focus on reducing damage is in PVP, and since this applies to all damage it is a little more useful than arcane fortitude. You can combine it with arcane fortitude and frost warding to gain some extra damage mitigation.

Arcane Instability
Description:
Increases your spell damage and spell critical strike chance by a percentage
Ranks:
Rank 1- Increase spell damage and crit chance by 1%
Rank 2- Increase spell damage and crit chance by 2%
Rank 3- Increase spell damage and crit chance by 3%
Usefulness:

Commentary:

This is another great talent with no real downside. It benefits all builds and play styles so if you have gone this deep you owe it to yourself to take it.

Arcane Potency
Description:
Increases the critical strike chance of any spell cast while clearcasting.
Ranks:
10% increased critical strike chance
20% increased critical strike chance
30% increased critical strike chance
Usefulness:

Commentary:
While this is a very nice talent to have it can be very unreliable. You'll clearcast once out of every ten casts, and this only increases your chance to crit, not guarantee it. You will only notice its impact during very long fights, so it is best suited for PVE builds.

Empowered Arcane Missiles
Description:
Your arcane missiles can an additional portion of your spell damage, but have their mana cost increased.
Ranks:
Rank 1- 15% extra spell damage bonus and 2% mana cost increase
Rank 2- 30% extra spell damage bonus and 4% mana cost increase
Rank 3- 45% extra spell damage bonus and 6% mana cost increase.
Usefulness:

Commentary:
This is something of a double edged sword. Arcane missiles is a great spell, and making it more powerful is great, but the mana increase can be a problem if you aren't careful. It will also only be good if you have a lot of spell damage, so if you are still in greens skip this one. But if you are well geared and don't have a problem with mana efficiency definitely take this.

Arcane Power
Description:
When activated your spells cause thirty percent more damage while costing thirty percent more mana. Lasts fifteen seconds.
Ranks:
One rank.
Usefulness:

Commentary:
This is often best combined with presence of mind and a powerful nuke like pyroblast. If you use with with trinkets you will have some of the highest burst damage in the game, or you can use it in PVE to give your DPS a shot in the arm. Just make sure you have a lot of mana once you use it because you will run out faster than you think.