On one of my many trips through various forums, I found the following set of rough guidelines for entry level gear to Karazahn.
While they are just that, guidelines, I feel it is a helpful list to see what you need to be able to enter Karazahn and do quite well.
Please note that this is NOT my list, but the mage section is accurate so I can assume that the rest is also. If you have anything to add to your class please feel free to post and I can change it accordingly.
Healing Classes
All Healing Classes need at least 7000 HP, 1000 +healing, 70 MP/5, 7000 MP
Paladins require 15% spell crit
Shamans and Druids require 8000 MP
Tanks
All tanks require at least 12,000 HP
Defense Rating: Paladins and Warriors require 490, Druids require 415
Armor: Paladins and Warriors require 12,000, Druids require 20,000 in bear form
Druids require 25% dodge rating, since they can’t parry or shield block
Paladins require 52.4% avoidance (parry + dodge rating) from spec and gear alone. 102.4% after buffs, and 200 spell damage
Warriors require 30% avoidance
DPS
All DPS require at least 7000 HP, and 6500 MP (if applicable)
Physical DPS need a minimum of 100 +hit, and 20% crit
Magic DPS require a minimum of 75 +hit, and 600 + spell damage (overall or specific)
Druids
To maximize potential output, druids should specialize in either casting or melee
Melee druids main focus stats are Str, Agi, AP, Crit, +Hit
Casting Druids main focus stats are + Spell Damage, +Spell Crit, MP/5, Int
Hunters
SV: 1550 AP, 27.5% crit
BM: 1700 AP
MM: 2000 AP
Mages
All mages should focus on +spellhit, Spell Damage, Spell Crit, in that order of priority
Frost mages should try to achieve 14% spell crit
Fire Mages should try to achieve 18% spell crit
Paladin
Paladin will need 25% crit and 1300 AP, as well as 4500 MP
Priest
Shadow Priests need +76 spell hit, around 7000 MP, and as much spell damage or shadow damage as possible. Spell Crit is not nearly as important as +damage
Rogue
All rogue specs require 175 +hit and 1300 AP
Shaman
Enhancement Shamans require 1000 AP
Warlock
The stat focus for warlocks is as followed: +hit, +spell damage, +crit, +sta, +int
+Crit is more important for Destruction locks, but all specs should have some +crit
Warrior
Both Arms and Fury Warriors need 95 +hit, and 25% crit
If Precision is not speced, then you will need closer to 143 +hit