Where to stand - Position yourself directly behind a melee toon (non-tank of course) so that they can easily turn and save you when you get bone spiked. Marrowgar has a huge hit box, so you should still end up pretty far from the center of the room.
What to watch for -
1. Coldflame! - Anytime you see the word 'flame' in a boss ability, it's pretty much a given that I'm going to tell you not to stand in it. This time, however, I'm taking it one step further... DON'T GET HIT BY IT. Coldflame is a talent that you will see coming... literally. It starts out directly under the boss and moves in a straight line toward it's target. The flame itself is no more than 5 yards wide, so when you see one coming in your direction, simply step to your left or right to avoid it. There's no reason to get hit by it, but even if you do it won't kill you unless you stand in it. SO DON'T STAND IN IT.
2. Bone Spike Graveyard! - Lord Marrowgar will cast a couple of these at regular intervals throughout the fight. The attack itself is a bone spike (hence the name) that comes out from the ground directly under its target and raises that target in the air dealing damage and preventing any action. It can't be avoided and must be dps'd down for the affect to be removed. Anyone who is impaled by this will need a good amount of heals until they are free from the spike. If you happen to be impaled, there's nothing you can do but call out for help in vent. Positioning yourself directly behind a melee toon will make life easier for everyone (and get you free from the spell faster).
3. Saber Lash. - This is Marrowgar's main dps ability and it's the reason your tanks will be stacking during this fight. You'll likely here people reference the bear boss from Zul'Aman when talking about this cleave, and that's because it's the same mechanic. The damage from the attack will be shared by your tanks as long as they stay stacked, so it won't be a problem for you to heal. If, however, your tanks are not stacked, the full force of this cleave will hit one target for huge amounts of damage. So keep your eyes open and be ready to heal big if a tank has to face Saber Lash alone.
Phase 2: Air phase
Where to stand - In this phase you should stand your ground as much as possible. Your raid should know they only need to move if a Coldflame is coming at them.
What to watch for -
1. Coldflame! - Yes, this is the same coldflame he casts in phase 1. However, during phase two Marrowgar will be casting this while mobile, which means they will be coming at you from different directions, criss crossing each other, and coming more often than in phase 1. Still, you will see them coming at you and as always, don't stand in them.
2. Bone Storm. - This ability is basically a whirlwind attack that lasts the duration of phase 2. While spinning and flying around Marrowgar cannot be taunted, so anyone can be hit by bone storm. The good news is that bone storm can be easily healed through, so there's no need to try and run away from it. Stand your ground if the boss comes at you with this attack. You should only have to move when he drops coldflame on you.
3. Kiters. - More dangerous than the Bone Storm attack are the toons who feel compelled to run from it. Running from this whirlwind will usually put a toon out of healing range. So while bone storm is easy to heal through, it can't be survived without any heals at all. Be ready to chase after any running fools.
4. End of phase 2. - Lord Marrowgar will target 4 toons with his whirlwind. After he attacks the 4th toon, he will stop spinning, wipe aggro, and begin phase 1 again. Any aggro wipe makes a fight harder, but when you add that to the fact that Marrowgar can drop aggro at any spot in the room, it makes this transition very dangerous for all in raid. Be mindful of where your tanks are so you don't end up in front of the boss after the phase ends. And do your best not to gain aggro while your tanks work to pick up the boss.
This fight is mostly a dps fight. Watch those coldflames and call out right away if you're bone spiked. If your dps is on the ball, raid healing will be very easy, so feel free to help out the tank heals if things are going well.