First, a few words on the trash leading up to Marrowgar. Most of it is pretty weak and a well-geared tank can often handle entire packs solo. That said, if things get out of hand, they'll get out of hand quickly. So don't get complacent.
There are also 4 bone giants (2 in each room, standing in the alcoves) that are triggered by Spirit Traps placed randomly in the two trash rooms leading up to the boss. As far as we've been able to tell in my guild, some of the traps don't appear until others have already been tripped. So it's entirely possible that an area that used to be safe will become hot. Be aware of this when running around to pull trash or heading back to the vendors in the front. If you have a Rogue in your group, they will be able to detect these and help you avoid them until you're ready. I do advise bringing down the giants (as opposed to having the Rogue disarm the traps) as they give 75 rep each. If you don't have a rogue to detect the traps, tread carefully and be ready to pick these guys up on a moment's notice. You'll get an emote when a trap is triggered and anyone in a small radius around the triggered trap will be slowed for a short time. When picking up the giant skeletons, it's important to immediately face them away from your raid. They Saber Lash and will instantly take out anyone who's not a tank or otherwise has high armor and a large health pool.
The biggest threat to tanks in this fight is Marrowgar's Saber Lash. Until you insanely outgear this fight, it's important that you and your fellow tank are on the same page. You'll want to be stacked on top of each other at all times to make sure the damage soak works properly.
Because of the size of Marrogar's hit box and the added bonus of his Saber Lash, you want to avoid letting him move towards the door as your raid is running in. This is a face-pull fight. You shouldn't toss any debuffs or damage his way until he's already aggroed. Once that happens, you're going to need to turn him to face the back of the room. Whether you do this by running straight through him and turning around or circle-strafing, make sure you and your partner tank are doing the same thing.
Update: The above pulling strategy still works, however my guild has started employing a different strategy that works better for us and may work better for you, too. Instead of running out to meet Marrowgar, you'll tank him near the entrance to the room with your back to the gate. Before the pull your raid should spread out along the walls in the room. This can be safely done without pulling Marrowgar as long as people hug the walls. Once Marrowgar is pulled to the tanks, the rest of the raid should circle around behind him.
As soon as Marrowgar aggros on either tank, you should hit Barkskin to help your healers during the initial positioning phase. Then get Faerie Fire (Feral) and Demoralizing Roar up as quickly as possible while, of course, Mauling to build up good initial threat. During the rest of Phase 1 you're going to be in your tried and true tanking rotation. Nothing special for you here.
On a timed interval, Marrowgar will cast Bone Spike Graveyard which will impale a random raid member on a spike. While he won't cast this on you while you're tanking him, it is good to keep note who it is cast on. The person impaled will take a decent chunk of damage up front and then continue to bleed 10% of their health per second until freed by the DPS, which means that they're going to suddenly be getting more attention from the healers. If one of your healers gets impaled, healing is going to become tight for a few seconds. You should be ready to hit Survival Instincts, chug a pot, or chomp a 'Lock-rock if your health drops below comfort levels. You can also use Barkskin or Frenzied Regeneration as a preemptive move to avoid getting to that point.
Coldflame is cast frequently during the fight. Typically this is cast at players standing out at range. However, if your raid does this encounter like mine, where everyone (except the Hunters) are bunched up in a tight group in melee range, most of the Coldflames are going to be cast right at the tanks. One right after the other pretty much. To avoid these you just need to dance left-to-right and back again out of them. Again, make sure you coordinate this with your fellow tank to make sure you stay stacked.
Finally, comes Bone Storm (Phase 2) which, despite what the tooltip says, only lasts for about 20 seconds. Marrowgar will go whirlwinding around the raid, targeting random members and dropping an 'X' shaped patch of Coldflame when he reaches them. As a tank, there's not much you have to watch for here. The damage from Bone Storm isn't anything you can't handle, so if he targets you, move out of the Coldflame and then start attacking him until he moves on. You should remain stacked with your other tank during this time and be standing near the center of the design in the room, only moving enough to keep Marrowgar in taunt range.
The part you really need to watch for is when Bone Storm ends. Marrowgar will aggro reset as soon as he stops spinning. (This is no longer true.) Your healers are likely going to be throwing around some heals to help recover, so you need to act fast. Immediately Growl and follow with Faerie Fire (Feral). I try to save my Barkskins for this transition back to Phase 1, as you'll start taking immediate damage, the healers will be spread out from running from Bone Storm, and they'll be topping off the rest of the raid during this time as well.
The rest of the fight is just alternating between the 2 phases. Easy as 3.14159.
On 10-man Marrowgar drops the Shawl of Neubian Silk which isn't horrid for Bears, although you're likely to have better if you're at this point in the game. There's also Marrowgar's Scratching Choker which, despite the wasted Parry, is still a nice Neck piece with the Stam and blue gem socket. On 25-man there's the Band of the Bone Colossus (which you'll most likely be passing on) and the very nice Frostbitten Fur Boots.
As with all my boss strats, I'm writing from the overall perspective of a Bear tank and what they will need to do. For you Cats reading this blog, I suggest you check out Dinaer's guides on Forever a Noob. They're written for Rogues but are a pretty good guide for melee in general and his gear drop suggestions (with the exception of weapons) will be spot on for Cats. If I have additional insight, I will try to add it in my blog. Here is his Lord Marrowgar strat.
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