As opposed to the relatively linear - or rather, entirely linear - Utgarde Keep and Azjol-Nerub, in the Nexus you're presented with three possible paths right off the bat. Straight ahead is the Red Dragon, Keristrasza, who is not only frozen inside a big block of ice but has a red name over her head and therefore is an enemy who must be killed posthaste. Given that the Red Flight seems to be on our side for this whole war, I can only imagine that she's not entirely under her own control. Perhaps the Red Dragonflight needs to work more with its members on resisting mind control?
Poor Vael :(
As I was saying, there are three paths. Straight ahead past the frozen Keristrasza is a massive cavern called simply the Rift, to the right is the Singing Grove, and to the left is the Librarium, which is a fancy term for what the rest of us call a "Library." Also, it's magic. And evil. Or something. There's probably a streamlined, quick way to do this, but as we were largely inexperienced here we naturally had to do it in the slowest, least efficient way possible. So we started off by heading right, to the Singing Grove. 
I'm not sure "Singing" is the term I would use, actually. I'd go with the "White and Glowing Grove," myself, because all the trees are not just bright white, they're luminescent. It's certainly a very cool effect, giving it a very ethereal feel. The whole Grove is filled with little non-elite lasher plants (you know the type), who when killed will revert to a seed form and become unattackable while very slowly regenerating health. They'll eventually fully return to life, which makes this essentially a very, very slow gauntlet-type scenario. There are also packs of wandering treants with an annoying tendency to use Tranquility and heal to full when low on life, and for the first few pulls we couldn't tell which one was actually casting it. (Hint: it's the Tender, so stun/silence/interrupt when they're low on health or you have a very long fight on your hands.)
This wing feels slightly too long compared to the other two, but at the end of it lies the boss, a (similarly white and glowing) crystal giant, Omorok the Tree-Shaper. Like most giants, he hits pretty damn hard. There are two abilities worth mentioning in this fight. The first is a standard spell reflect, though unlike most bosses, this actually reflects a certain number of charges rather than having a duration. So ... well, you can't just wait it out. To save yourself and the healers some trouble, I recommend just tossing a rank 1 nuke for convenience's sake.
The other ability is a fun one. Ever so often, Omorok will shoot little icicles out along the floor in a few random directions. The little icicles quickly become big icicles, and shoot anybody standing on them up into the air, simultaneously dealing a few thousand damage. They aren't all that hard to avoid, but it's a fun ability and I probably enjoy getting launched up into the air entirely too much for my own good. But hey, I have Slow Fall, so ha!
We reduce Omorok to rubble and backtrack, killing the regenerated thrashers in our wake. Thankfully, we don't have to go all the way back to the middle, as there's a shortcut about halfway back that takes us right to the Rift. The Rift is a gigantic cavern that's probably one of the cooler sights I've seen in a dungeon yet. We aren't the only ones fighting here; the Blue Dragonflight is busy fighting off assorted ... things ... from presumably, uh, some sort of magic rift. That's as far as my best guess goes, anyway.
Amongst the many enemies in the cavern are actual smaller rifts that don't do any damage by themselves (as far as I saw) but that constantly spawn rift wraiths until destroyed. I'm not quite clear how stabbing a magical tear in space-time closes it, but eh, I'm not going to question it. These should be the first targets whenever you encounter them, or else you might find yourself overwhelmed.
There are also larger arcane elementals that drain mana. I hate mobs that drain mana. I hate fighting three mobs that drain mana at once. That's just annoying. When you kill these particular enemies, they have the whole "Loose Mana" regeneration that we've seen in Karazhan, which is nice, but it doesn't make their mana draining while alive any less irritating.
At the very end of the Rift is the boss of the wing, Anomalus. There's some sort of rift-serpent-thingy flying around behind him, so I advise pulling him down onto the platform in front of him lest you bite off a bit more than you can chew.
Further evoking memories of the Menagerie in Karazhan, the fight with Anomalus bears some similarities to fighting The Curator. To be fair, it's really just a cursory resemblance: every so often, Anomalus will spawn a rift similar to the ones you fought on the way here. The rift does a bit of damage and spawns more rift wraiths - furthermore, he's invulnerable while it's active, so you want to down those right away. It's not a particularly difficult boss battle, but I enjoyed it nonetheless.
