In my opinion? Lord Marrowgar is the perfect first fight for ICC. There is enough going on in the fight to keep your mind going, yet everything is simple enough to look out for and react to. A enjoyable fight and great first experience at 3.3 raiding.
Where will the boss be?
The boss will be tanked where he is, in the center of the room, by your 3 tanks! During his Whirlwinds, you are going to find you go OOR of your tanks VERY easily. I would advise marking them, so you have ready heals when he starts attacking again.
Where should I stand?
As a ranged you need to be spread out in a semicircle around Lord Marrowgar. The idea is that you stand far enough to be able to dodge Cold Fire as it spawns out of his hitbox, and close enough for melee to lend a helping hand, lest you get hit by a Bone Spike. You also want to stay away from your tanks, as you don't want be eating any Saber Lash damage!
What should I look out for?
Saber Lash - This is a melee attack that lord Marrow will deal to your tank and your tanks 2 closest allies (In 25 man, this would be your other 2 OT's). A very fast, powerful attack that will put stress on your HoT's. If you have been assigned to tank healing, you are going to want your Glyph of Nourish, complimented by your Rej, Regrowth and Lifebloom HoT's. Lord Marrow is also abit sneaky and will usually use this ability RIGHT after a Whirlwind, So another friendly reminder, keep an eye on your tank, and don't let them go out of range.
Cold Flame - Lord Marrow is going to be pumping these out during the whole encounter, However they are easy enough to avoid. The effect is of a blue strip of flames that will spawn at melee range and move outwards in a straight line. Melee won't take damage from it if they can move fast enough when it spawns on them, and ranged should be far enough to be able to dodge it as it travels outwards. It will do moderate damage to people to get hit by it, however a simple Regrowth or Rej/Lifebloom should be enough to top them up.
Bone Spike Graveyard - During the encounter Lord Marrowgar will hurl a boulder in a random direction, hitting anyone in it's way with a large spike. This spike will be targeted and taken down by ranged DPS but in the meantime, the Spike's victims will be taking damage equal to 10% of their max health, every second. Give them a Rej and Regrowth, and if they dip too low in health, use your Nourish. If YOU get hit by a spike you will find yourself out of action,and unable to heal. Let your raid know over vent or have a Macro ready, so you can warn the other healers to cover for you.
Bone Storm - Every 30 seconds, Lord Marrow is going to cast Bone Storm, or as you would know it, Whirlwind. He has no aggro table while this is being cast, so he will simply target a random player, move towards them, dealing damage to everyone in his path, before changing to a new target. He will target a maximun of 4 players during each Bone Storm, and the closer you are to him, the more damage his whirlwind will do to you. On a brighter note, the tanks will have a break from soaking up the Saber Lash, as he does not use this ability while he is WWing around the place. Use this time to top up the tanks and re-HoT them, and use your Rej blanket techniques for the rest of the raid (5 Rej's + 1 WG, rinse and repeat)
Any healing Assignments?
We had 6 healers for our first night in 25 man ICC, So I assigned 1 healer for each of our 3 tanks (2 pallies, 1 druid), 2 raid Healers (1 Disc priest and 1 Druid), and 1 "throw-around" healer (Myself) to be on raid healing, whilst taking over other healers assignments when they got spiked, and out of action.
When should my raid pop Heroism/Bloodlust?
I would suggest popping it right at the pull or just after the first Blade Storm, after your MT has gained a comfortable amount of threat.
Welcome to 3.3 raiding guys, Goodluck!!!
Wow Slang Dictionary
OOR - Out Of range
MT - Main Tank
OT - Off Tank
Rej - Rejuvenation
Wg - Wild Growth
WW - Whirlwind
HoT - Heal over Time