Enjoy the read, pasted below as well.
Hit rating, expertise rating, and spell hit rating are one of the best ways to increase DPS against raid bosses. As raid bosses are considered level 73, you have a much higher chance of your attacks missing than normal. Additionally, hit rating tends to be a "cheap" stat when gear is itemized, meaning that an equivalent amount of hit rating usually does more for your DPS than the same amount of critical hit rating.
However, eventually you have enough hit rating to never miss the boss, and accumulating more hit rating is unnecessary. This is usually referred to as hitting the "hit rating cap".
Spell Hit Rating
Base Spell Miss: 16%
Spell Hit % = spell hit rating / 12.6
Spells actually have a 17% chance to miss, but you cannot eliminate that last 1%, so the effective cap is 16%.
The following is the amount of spell hit rating you need to minimize misses for raid boss fights, depending on your class and talents.
w/ Elemental Precision (Fire/Frost): 164
w/ Arcane Focus (Arcane): 76
w/ Suppression (Affliction): 76
Note that a warlock might still want extra spell hit rating for Shadowbolt and Soulshatter, in which case Suppression becomes redundant.
w/ Shadow Focus (Shadow): 76
w/ Focused Power (Smite, Mind Blast, Mass Dispel): 152
w/ Shadow Focus + Focused Power (Mind Blast): 26
w/ Balance of Power: 152
w/ Elemental Precision: 126
w/ Nature's Guidance: 164
w/ Elemental Precision + Nature's Guidance: 88
w/ Precision: 164
Mage/Priest/Shaman Draenai Aura: -12.6 spell hit rating needed
Shaman Totem of Wrath: -38 spell hit rating needed
Base Melee Miss (Dual-Wielding): 28%
Base Melee Miss (2H/1H+Shield/Special Attacks): 9%
Base Ranged Miss: 9%
Hit % = hit rating / 15.8
For example, as a paladin wielding a two-handed axe, I need to get 9% worth of hit rating, which is equal to 9 * 15.8 = 142 hit rating, in order to never miss the boss. Hit rating beyond 142 is not useful. After hitting 142 hit rating, I should concentrate on other stats like attack power and critical hit rating to increase my DPS.
For dual-wielding, the 28% miss chance applies to both Main-Hand and Off-Hand weapons. However, special attacks, such as Backstab, only have a 9% miss rate. For most classes though, regular melee damage is a significant portion of your total DPS. For a more involved discussion of this, see here.
Unlike spells, when you reach your hit rating cap, your attacks will never miss. Melee attacks can still be dodged or parried. Melee attacks can only be parried if you are in front of the mob.
For tanks, please note that Taunt uses melee hit rating to avoid misses/resists. However, testing at Elitest Jerks has shown that Taunt has a much higher miss rate (~16-18%). It is unknown whether this is intended, and it may vary by boss. For example, Nalorakk in Zul'Aman is more vulnerable to Taunt than other bosses.
The following is the amount of hit rating you need to minimize misses for raid boss fights, depending on your class and talents.
w/ Precision: 363
Note that current wisdom holds that rogues should attempt to reach the cap to maximize their damage.
Rogue (Special Attacks)
w/ Precision: 63
w/ Surefooted: 95
w/ Dual Wield Specialization: 348
w/ Nature's Guidance: 395
w/ Dual Wield Specialization + Nature's Guidance: 300
Note that current wisdom is that a Shaman only needs about 9% hit, and should not focus on reaching the cap.
Shaman (Two-Handed/One-Hand + Shield/Special Attacks)
w/ Nature's Guidance: 95
w/ Precision: 395
Note that current wisdom holds that warriors should not focus on hitting cap, though they should get at least 9% to ensure that their specials always hit.
Warrior (Two-Handed/One-Hand + Shield/Special Attacks)
w/ Precision: 95
w/ Precision: 95
Hunter/Paladin/Warrior Draenai Aura: -15.8 hit rating needed
Druid Improved Faerie Fire: -47 hit rating needed
[Spicy Hot Talbuk]: -20 hit rating needed
Base Melee Dodge: 5.75%
Base Melee Parry: Varies by boss, at least 5.75%
1 Expertise = expertise rating / 3.95
1 Expertise = -0.25% Dodge and -0.25% Parry
Expertise rating is a stat introduced in Patch 2.3. It replaced Weapon Skill rating. Expertise rating reduces the chance of your melee attacks being dodged or parried.
Melee DPS should always be behind the boss to eliminate parries, so they only need to worry about dodges. Expertise rating is essentially as valuable as hit rating for melee DPS.
Because tanks are in front of the boss, and can have their attacks dodged and parried, expertise rating is twice as valuable as hit rating for them.
When you reach your expertise rating cap, your attacks will never be dodged. Melee attacks can still miss. If you reach both your hit rating cap and your expertise cap, your melee attacks will never miss or be dodged.
The following is the amount of expertise rating you need to minimize dodges for raid boss fights, depending on your class and talents.
w/ Weapon Expertise: 52
w/ Defiance: 68
w/ Weapon Mastery: 60
w/ Defiance + Weapon Mastery: 37
w/ Combat Expertise: 72