Before you even read the numbers, you should consult Anna’s blog: Am I Ready to Heal Naxx?

Note: These numbers are good for both Naxx 10 and 25.

Note 2: Your mileage may very. Experiment with different raid combinations to find out what works best.

Tanks

Health: 25000 unbuffed
Defense: 540 (Crit immunity)

Note: Druids will have a higher health pool. 30000 health is a good number to aim for.

Melee DPS

Hit: 9%
DPS output on average: 2000 DPS

Obviously the more the merrier.

Caster DPS

Hit: 17%
DPS output on average: 2000 DPS

Note: Both percentages assume you are completely naked and lacking in hit-increasing buffs. You can find your hit percentage by mousing over the hit rating on your character screen.

Healers

Spellpower: 1550
Mana regeneration: 700

Paladins mana regeneration: around 200 with 25% crit is a good start

Shamans may have slightly lower mana regen.

Treat these as guidelines! Use your discretion. If you can handle a few heroics under your belt, then you’re ready to give Naxx a shot! Don’t expect to be killing Kel’Thuzad or anything right away. Know your limits.