So, Malygos, the final boss in 3.0 Wrath of the Lich King. It's a good challenge. My guild has mostly breezed through Naxx and the raid bosses; I think the hardest thing for us so far has been Grobbulus and that got a lot easier once our raid leader found a better positioning. Malygos is a bit more of a challenge, with all three phases posing different problems and a tight enrage timer requiring precise execution. We still haven't killed him, but now that we've gotten him down to 4% in normal mode I think I understand the 10 man tanking problem pretty well.
In phase 1 the main tank has a lot of jobs to do. You need to grab the boss quick. You need to stay alive, he hits hard. You need to generate a lot of threat: your DPS is going to have 100% damage buffs and you won't, so threat capping becomes a significant problem. And finally, most importantly, you have to position the boss right.
Positioning is hard. He has to face away from the raid at all times, of course, for the breath. But you also need to move him so that the sparks that spawn pass through the middle of the room on the way to the boss. You want to be sure a spark never reaches the boss. Not so much for the damage increase (it doesn't hurt too bad in 10 man) but because your DPS needs to kill those sparks themselves and stack them to get 2x damage buffs at all times. If you don't stack the spark buffs you'll never have enough DPS to get through the fight before the enrage timer. So you're constantly watching for sparks, dragging the boss around (facing away!) to make sure the sparks have to go through the middle of the room on their way to you.
Threat is hard too. In my attempts today I had our frost mage and shadow priest both right on my heels, or above me. Simple things help. Use Berserk early and often. Get misdirects / tricks of the trade. DPS all the time: you can melee the boss a lot longer than you'd think, including during the Vortex if you're lucky. Even with all that it wasn't always enough; my mage and I worked out a thing where I'd let him pull aggro during the takeoff and vortex phase, then I'd taunt off when he landed and the mage would go invisible to drop aggro. Risky, but it worked.
The whole challenge of phase 1 is getting through it fast; you really don't want it to take more than 4 minutes. Phase 2 is a lot easier to tank. You have to pick up two adds at the start, beyond that it's just running around and keeping yourself alive by getting under shields. If your party is melee-heavy you may find yourself on the ground with nothing to attack and no discs to go flying on. If so, make yourself useful and start healing. Grab a disc when you can and go flying-kitty, it's a lot of fun.
Phase 3, oh, phase 3 is heartbreak. It's one of those damn "you're not a druid anymore" fights where you have to master some random set of new mount abilities. Fortunately you can practice the dragon ahead of time in the daily quest. I suggest raid leaders take their raid groups to the daily quest area before the first couple of attempts just to try it out in a group. If you're in a DPS role in phase 3 it's easy. Press 1 two or three times to get some combo points, then press 2 to stack your DoT. Repeat. Every time Malygos starts charging up Surge of Power don't press 2; instead build up a couple of combo points and make sure you have 25 energy. If the Surge is on you then watch him cast it, then time it so you activate your shield (5) right before his cast goes off. If it's not on you, go back to DPS. Not too difficult once you get the rhythm of it, but there's a lot going on and its disorientating so it takes some practice.
All three phases have to be executed well in order to win. Phase 1 and 2 get easier, after a few hours our 10 man was able to get through them reliably. But you have to do it quickly, you want at least three minutes for phase 3. And then you have to do your dragon thing well, which takes more practice. I'm sure he's going to fall for us soon.