So I piled myself into a 10m RS PUG last night (the loot table bores me, but I wanted to see it once) and I immediately warmed up to the zone. The group I was in wasn't the sharpest bunch of tacks, but we killed the three mini bosses and had a bit of time on Halion. Here's my initial thoughts.
I love the trash. Just love it. Bunch them up and AoE for great justice. Then watch your tank get folded, your DPS die in a Blast Wave, and start flying back on your spectral griffon. No, these trash take a little coordination (pull one out, etc) and focused fire. Fire Aura is your friend here (shocker, I know).
Mini Boss #1
This is the Draconid standing on the left when you enter. I don't know what he does, but one of our tanks got folded like an origami swan that you just jammed into your pocket. I think there's some kind of switch that should go on here - even after he forms a copy of himself at 50%. Either way, a tank died inside of 1 GCD - so it was epic.
Mini Boss #2
This is a little drake sitting off to the right in the instance near some water. I'm fairly certain that the pool in Ulduar is still the only water in Northrend that extinguishes fire - so don't bother standing here. The drake uses the standard "Don't stand near someone with a mark" mechanic and puts a Conflag on them. Spreading out and staying away from the drake's face work well here.
Mini Boss #3
This is one of those squat looking draconids, similar to the trash you've been pulling. Using a cleave and an armor debuff, you have to swap tanks after every 20 or 30 seconds (after the adds die is a good time). Uses a stand in place fear to add a moment of danger, especially when the spriest uses fear ward on themselves and he decides to ignore the feared tank. Tip - don't back up through the smokey/fiery wall or you'll be out of LOS from your healers.
Halion uses the "Don't stand in fire", "Don't stand in circles of death", and "Don't eat lazors" mechanics. He also has the "Raid must work together to DPS both versions" mechanic. Halion also has a hit box that makes Onyxia's look like a pinhead. Ahune's hitbox may be slightly smaller, but just slightly. I'm fairly certain I saw Rhonin DPSing from Dalaran.
Short version of the fight is this.
100%-75% - DPS like you mean it. Don't stand in the fire, watch the tail whip/cleave, and cleanse off the fire debuff AFTER someone runs away from the raid. Honestly, letting it stack 2 or 3 times is better than cleansing this off in the middle of the raid.
75%-50% - Everyone but one tank and one healer into the portal. Large hit box here still, and the same mechanic for cleansing as Phase 1 (the difference being if you stand in/near it you will get pulled in to the center of it instead of pushed away). There are two orbs spinning around the outside of the room that will occasionally join with a beam that will one shot you - easily the deadliest part of the encounter.
49%-loot - Split your raid between the shadow and the corporeal, and just burn him down. You want your DPS to be as even as possible (so I've heard), and just avoid the same mechanics as above.
Uh - so why is this great again?
OK, maybe the encounter isn't all that and a bag of Lich King chips (BBQ flavored), but it is something new that will be a challenge for PUGs everywhere. I imagine by next week you won't be able to get into a regular RS 10m kill without a "5700GS + achiev or no invite", even though the fight isn't overly difficult. It certainly won't hold my attention for more than a few kills, and I'll probably never step foot in it again once I get a 10m or 25m kill. The gear just isn't that exciting for my Paladin.
The encounter certainly wouldn't be enough to keep me from canceling my account (if I was going to), and won't be enough to stave off the masses until Cataclsym's release in Oct/Nov. With the closed Beta opening up today, we'll probably see open Beta in August, and the release event coming in late September or early October - with a release to coincide with the weekend after Blizzcon (I'm guessing). Unless I misread Halion's shadow, we've still got 12 more weeks of winter ahead of us.