I got in on the Dragons of Nightmare, meaning that I missed Ursoc and thus don't have a rundown of him yet. Once I do (I probably won't bother running it again this week on my main as the gear is all at least 5 points lower than everything he's got) I'll edit this post and put in stuff about him.
Update: I was able to clear the full wing on my Death Knight (and even got an 845 relic for my axe!) so now I can go over the full wing.
Ursoc is one of these bosses that has two debuffs, a bit like Gruul in Blackrock Foundry or Nalorakk in the original Zul'aman (yeah, that's an old reference.)
I think it's more or less a damage race - not sure there's much for DPS to worry about, but I'll have to run it on a DPS character to know.
Tanks will get debuffed by Overwhelm and Rend Flesh - the former increases damage taken by the latter and I believe stacks up. In theory (and I'm sure on Normal and higher difficulties) you'll need to ensure that one tank is taking Rend Flesh (which is a bleed) while the other is getting the Overwhelms (and possibly swapping who gets it when Overwhelm stacks up high, as I think it also increases its own damage but by a much smaller amount.)
Still, this is a "stand your ground" kind of boss, at least on LFR, so it's fairly simple.
Dragons of Nightmare:
This fight will remind you of the Four Horsemen if you ever did it in Wrath's Naxxramas (I'm given to understand you actually needed eight tanks in the original Naxxramas, which is pretty insane.)
The main crux of the fight is that each dragon will radiate an aura that causes a stacking debuff. If this hits 10, you'll be stunned for I believe 30 seconds, which means you're probably going to be dead.
While my co-tank preferred to move to the dragons rather than dragging the dragons across the room, I imagine the better strategy would be to have each tank stay in place and taunt the other dragon (well, they'll have to move to get in range.)
Basically, you want to have every raid member and dragon on one side of the room or the other so that no one is getting stacks of the debuffs from two dragons. If you taunt on 6 stacks, you'll have time for the other dragon's stacks to fall off without paying much of a penalty.
The big wrinkle, however, is that while I believe Ysondre will always be down, the other three dragons will rotate - so if you're currently on Ysondre, you can't simply taunt the other tank's dragon when it's time to switch.
The individual abilities of the dragons escape me, as I was more focused on this, but I'm sure I'll have more thoughts on this once I take a DPS character in.
Cenarius is a two-phase encounter, with phase two starting at I want to say 40% or 30%.
During phase one, you can actually just have one tank on the boss while the other gathers adds. I happened to be on the boss, but generally I think you want to keep the adds off the boss, as they might buff or heal him.
Everyone will get a stacking debuff that increases damage taken by the adds (I believe they do some AoE moves so that's a concern for everyone.) Periodically, Malfurion will purify a patch of ground and an add, which will allow them to aid you in the fight as well as creating a big green patch. Stepping inside this will reset all your stacks of the big debuff, but it will also diminish the size of the patch, eventually causing it to disappear.
Update: If you're tanking the adds on phase one, there are a few kinds you'll get - dragons, dryads, and ancients. Malfurion will purify one of these (I can't recall how one chooses for him) but the other two need to be picked up. The dragon has a breath attack, so you should point him away from the raid. The ancient has an AoE stomp and the dryads have a spell that should be interrupted (you get two at a time, I believe.) There might also be a fourth kind of add, which is a group of wisps, but I think the one time we got them when I was tanking adds we purified them, so I don't know what they do.
On phase two, I believe the adds stop spawning, but now Cenarius will start hitting the tank with a powerful spear attack that leaves a debuff. On higher difficulties I think this creates a need for quick taunt-swapping, but on LFR I was able to handle a few stacks while my co-tank finished off an add from phase one.
Defeating Cenarius will open up the way into the Rift of Aln, but that wing isn't open yet, so you'll have to wait until... next week? Honestly I was surprised that this one was open already, as I figured we'd get it two weeks after wing one.
Overall I'm really liking the Emerald Nightmare raid. LFR's low gear rewards (though clearly they can warforge or titanforge into more attractive pieces for those of us who have mythic dungeon gear) and the lack of any long legendary quest grind means I'm somewhat less incentivized to ensure I do a full clear each week, but it does make me eager to bring in lower-geared alts and definitely to get my guild running this on Normal.