Well, I don't know, but this seems like a mix of over-confidence, whishful thinking and carefully staged image perpetrated by the major commanders that seems to characterize every mass military engagement (read epic battle, or slaughterfest, depending on personal perspective :P) between the bronze age and the advent of semiautomatic firearms.

Basically the same thing we raiders do when we're attempting a new boss... "He's soooooo going down tonight". Granted, we have unlimited(or not) attempts, but it's the same enthusiasm before the actual first contact. Even if it might take us a few weeks in the end...

That's the kind of trailer that would only be awesome if:

a. The Lich King had spent most of this expansion's storyline scoring unexpected and morally devastating victories against the forces of justice, rather than getting his tail whipped at almost every turn. Sorry, but the clichéd, "He raises all your fallen as Scourge," is neither moving storytelling nor does it have any noticeable effect on the progression of the gameworld.

b. If the Lich King had spent the whole expansion getting his tail whipped only to turn around and kick all our asses in both the dungeon and raid encounters, ending the expansion victorious and with us fleeing his lands, barely escaping with our lives.

As it is, the finale of the Icecrown raid better be pretty amazing.

So I did the first part of the Shadowmourne questline, since umm, that's the only part that can be done :P

When you enter the Frostmourne Cavern Arthas appears, calls you predictable, saying how he knew you would come, he then locks you in ultimate chains of ice and summons about 200 Undead. surrounding you, and throws Light's Vengeance into the corner of the room, he leaves with a Vrykul Necromancer constantly summoning more ghouls, and the ghouls slow you as you make your way over to the Mace. I imagine that killing the ghouls so you can actually loot the hammer is much easier as a Paladin or DK with their AE >.> Anyway you get to the mace, and it shines with the light and annihilates alll of Undead in the room. Then when you try to loot it, the Undead Necromancer comes back to life and you have to kill him, he has about 150k HP or so. He keels over and you can claim your prize, again I imagine this is easier on Paladins or DKs since they can heal, I killed him and only had 3% HP left. :P

Dialog:

*Ultimate Chains of Ice cast on player*
Lich King: So predictable...
Lich King: Did you truly expect to fullfill Mograine's task unopposed?
Lich King: You both shall suffer as his father does...
Lich King: ...in eternal unrest!
*Summons 200 Undead*
Lich King: Die well, fool.

That makes the 2nd time that Arthas has conspicuously mentioned owning Alexandros' soul. It could just be that he has a particular hatred of the Ashbringer, but I have to wonder if Blizzard isn't hinting at some involvement from him in the Arthas fight.

Note: I thought the Shadowmourne questline couldn't be started unless you were Friendly with the Ashen Verdict. My mistake, it seems.

On Mograine, I think I'll go with the "hate" (and gloating) theory. Perhaps it's just out of caution of seeing the final fight turned into a "A wild Mograine appears", character-spawning experience. Sure, papa Alex is an important character and one of the most significant of Arthas' victims, but hasn't he had his spotlight? Not every great hero from the past has to return in order to remind us how great he was/is/might be in the future. I'm an adept of tragic fates, though not for everyone. For those that do get one - in a plausible and well-done way, preferably - they should just stay dead. What befell Alexandros is not a limbo, like what'll supposedly happen to Bolvar. An unknown fate isn't the same as death in front of the audience (Alexandros, again). And the fact that it was perpetrated by his own son, coupled with the awesome retribution, means there's little chance any future return by Alexandros has anything other than a "tutoring" ghost is very unlikely to surpass his death and not come out as cliche'd.

From my vague memory it was my understanding that Orgrimmar was going to be partialy destroyed, with a new Orgrimmar being built with help from the Goblins. Would make perfect sense if these caverns were to errupt upon the event of the Cataclysm.

I am still holding on to the idea that the now pure ashbringer held by Tirion is going to play some big role in at least the Arthas encounter. Up until now boss fights have all been 'figure out how it work + use this strategy + don't stand in fire = loot'. If somehow something goes terribly wrong for the heroes of Azeroth in the LK fight, and Tirion/Darion have to step in to save us it would make for a very cool ending to the Frozen Throne.

This big bad villain move of the Lich King confirms my theory that part of the final encounter will involve sharks with laserguns on their heads. Maybe they'll fly too.

Emeraude is friendly with the Ashen Verdict. Doing the first four bosses on 10 man and 25 gets you well into friendly.

I thought it was due to Garrosh's view of not only the Horde, but the new "war-prone" status quo. Perhaps I was basing myself on some other bases' changes, like the one at Ashenvale's beach, which was explicitly said to have suffered transformation for military reasons.

I doubt it will really be "next to impossible", at least not the normal version. That would contradict with Blizzard's premise of the raid content being accessible to more people. The heroic version, perhaps.