In this section we will talk about Arcane specialization in the Wrath of The Lich King expansion, specifically in the 3.3.5a patch.
It's the most powerful specialization for short encounters (0 to 90 sec) but the hardest to play.
First of all, you are a mage, so your role is to dps, and in order to do it you need to stay alive. Mages have huge DPS potential, but they are one of the most squishy characters in the raid.
Let's talk about the Arcane mage especially:
As an arcane mage, your main features will be your amazing burst potential and your almost non-existent moving capacity.
So your dps relies on having flawless placement and to be able to exploit the few times you will be able to move without losing DPS.
You have a very short casting time so you will be very useful on bosses where your raid needs a quick switch. Though you will have to stay still.
Not moving is mandatory because you have no DoTs and no instant spells in your rotation which create a GCD, so every movement will be a net loss !
As an Arcane mage you can be as competitive as a Fire one, especially on a short amount of time, but the technicality will be to properly manage your mana. Indeed, being out of mana right at the end of the encounter would say that you have exploited your dps potential to the maximum.
To conclude this small introduction, I'll add that in my opinion playing Arcane is harder than Fire- you need to perfectly master your fight and have exceptionally cold blood to do well. For sure it's the more efficient spec for an early 80, but do not underestimate the difficulty of playing this specialization. It's not complex but it requires good concentration.
If you have a powerful enough group, these will be the selfish rules you will have to apply to increase your DPS even more:
Holding threat is the job of your tanks.
Keeping you alive on little damage such as puddles (the time you need to move without losing dps) is partly your healers' job.
But remember that if you apply these selfish rules it's your job to do amazing DPS and an ultra reactive switch on targets who need to be killed !