Heroic: Storming the Citadel (10 player)

For differences between this and the regular counterpart are as follows:

Note: All the bosses get more hp, +10% I believe. Lady Deathwhisper will have the same amount of mana, but double her hp

Lord Marrowgar:

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• His flame now lasts for 8 seconds
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• He will still cast bone graveyard while bone storming
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Nothing too difficult, but watch out for Bone Spike + Bone storm.

Source : http://www.wowwiki.com/Lord_Marrowgar#Heroic_Mode

Lady Deathwhisper

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• Immune to Taunts
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• She will Mind Contol 1 player at a time
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• 1 an add will continue to spawn during P2
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• Vengeful Shades will explode in a 20 yd radius
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I would say this is probably she is the hardest

Source : http://www.wowwiki.com/Lady_Deathwhisper#Heroic_Mode

Gunship Battle:

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• Mage has more health.
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• The rockets (the red circle that glows on the ground) now knocks you back 7 yds if you get hit by it.
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• Saurfang/Bronzebeard now gets an extra stacking
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• 12% damage, instead of 10%.
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Still the same faceroll it is on regular mode.

Source : http://www.wowwiki.com/Gunship_Battle#Heroic_mode_2 & the +12% stacking buff is from personal experience.

Deathbringer Saurfang:

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• 5 seconds after a blood beast spawns, it gains "scent of blood", decreases movement speed of the raid by 80%.
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• Blood beasts can now 1 shot any non-tanks.
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• He heals for 20% when someone with Mark of the Fallen heals, instead of 5%.
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• Significantly more Blood Power gain.
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Source: http://www.wowwiki.com/Deathbringer_Saurfang#Heroic_Mode

Have fun!

These kill achievements didn't seem to count for me until I was in a 2man raid group.