If Matthias was bound to the heart all this time, wouldn't the heart's destruction release it? Not as in having a ghost child wandering the battlefield when we're storming the Citadel, but rather, weakening Arthas somehow.

That’s exactly what happens when you finish the quest and Tirion destroys the heart: Arthas instantly loses 70% hp and writhers in pain on his knees. Queue more dialog then Darion Mograine then opens a death gate for you to escape with before Arthas musters the strength to react.

From the point of view that the blues have been pretty open about their philosophy that everyone will be able to encounter The Lich King. Don’t get me wrong, I’m not saying the storyline will even come close to ending there – but I think there will be enough interaction and “closure” of sorts in there give some satisfaction the non-raid population, which is a big part of the Wrath ethos. That’s why I lean towards the theory that the Jaina and Sylvanas stories with Arthas will be closed in the 5 man while the demise Arthas himself is dealt with in the raid. Non-raiders help a faction leader get the closure they need, raiders deal with the final confrontation. It's win-win for all involved.

One doesn't need to exist, it's just that we've been lead to believe that one will.

World of Warcraft - Lore & Storyline discussion [SPOILERS]

I have no proof, but my guess is that it's linked to both the sub-zone in the Citadel and the survival of Bolvar / Saurfang; we find that the sword is capable of trapping its victims in a way that means various people are still reachable in some way. When is the first point in ICC at which we know Bolvar is alve?

Uhh.. we didn't? I guess fetching the Head of Kael'thas for the Shattered Sun Offensive was all in my imagination then!

No people are making jokes about him coming back from the dead. When have we seen him alive since the end of MGT?

Kael'thas-related jokes derive from the fact that his return in Magister's Terrace was mostly unexpected and sometimes seen as a show poor imagination by Blizzard. To top it all, he had the "Tempest Keep was merely a setback" line, which was pretty much gold an is ever since used whenever Blizzard decides to bring back characters (enemies, mostly) who were presumed dead. Like Anub'arak - although Anub'arak's return in a raid instead of a dungeon, unlike Kael'thas', was something most of the WC3: Frozen Throne players wanted very much.

I can't really see Illidan returning. Like someone said above, we have a bunch of characters with more than enough motives to want the Lich King dead, there's little need to bring back one whose image is that of a madman. Also, in his mind, he's defeated Arthas...

Has it been confirmed that the Frostmourne sub-zone is from the Icecrown raid? It could the our way of uncovering Frostmourne's secrets. The idea by Li Master that we'd find Frostmourne's victims trapped inside the sword seems very interesting. Plus, just running from Arthas is scarce for the final encounter of such an important dungeon.

Edit: Just so it's clear, I liked Kael'thas' fall and return a lot. Not only was it a demonstration of what happens when you start using fel magic for fuel, the crystal-in-chest detail was, to put it simply, awesome.

Had forgotten about the hint after we killed him at Tempest Keep. Truth be told, then, most of the fuss was because of Blizzard turning him into an evil character (which was way before MGT - there was clearly something wrong right from the start of TBC). Still, Kael became a staple of "villain comebacks" for WoW, and his "merely a setback" the signature for all other comebacks.

Most serious criticism was really about the amount of heroes that turned evil in Warcraft.

Hardly. You might have noticed that so far there hasn't been any sign of Kael reappearing subsequent to the forcible removal of his head in Magisters' Terrace. Kael jokes rely on his "Tempest Keep was merely a setback" moment in that particular instance. As TSplodey wrote, that return should not have been a surprise to anyone who was present when the Verdant Sphere quest item from Tempest Keep was handed in, at which point it was explicitly stated that Kael'thas was alive, and he even had a muahaha insane villainy moment pronouncing his allegiance to Kil'jaeden from the distance.
Picking three main reasons for the popularity of the "merely a setback" meme, even though other bosses have made returns that might be considered less probable, I would go for these:
-the farcical nature of Kael'thas' speech and appearance in Magisters' Terrace
-the somewhat camp character of Kael'thas in general
-chance are that more people have encountered the MgT Kael than the event in Shattrath, and for the latter group it would usually have been a relatively distant memory when the Sunwell patch was released.

An Argument could be made for the permanent death of the Scourge, because Cataclysm wise, Blizzard can remove the scourge from landscape of Azeroth (save Northrend), so that in a chronological order, the world post Cataclysm will be scourge free.

The last time Lich King lost control over his army, the Forsaken faction was born, being freed from his control and that was thanks to having a powerful Banshee lead them. The Scourge lack a will of their own, they follow their orders and are loyal to their masters.

The elimination of Lich King would mean there is nobody left to control them. So, if you go in chronological order (which will not be the expansion order come Cataclysm), an Azeroth infested with the Scoruge carrying out Lich King's commands would not make much sense.

That's something I've always disagreed with, ever since Kael 25 was able to be killed (long before patch 2.1) a Shattrah-wide script started after handing in the quest item that showed he was alive. He didn't return until patch 2.4, and even when he did it fit in perfectky with Blizzard's original script:

<Silence descends upon Shattrath>
<A'dal's thoughts invade your mind>
<Kael'thas Sunstrider has been defeated by (player) and his allies>
<The time to strike at the remaining blood elves in Tempest Keep is now. Take heart and let A'dal's song of battle empower you!>
Kael'Thas Sunstrider Yells: Your monkeys failed to finish the job Naaru. Beaten, but alive... The same mistake was not made when we took command of your vessel.
Kael'Thas Sunstrider Yells: All for what? Trinkets? You are too late. The preparations have already begun. Soon the master will make his return.
Kael'Thas Sunstrider Yells: And there is nothing you or that fool, Illidan, can do to stop me! You have both served me in your own right - unwittingly. Lay down your arms and succumb to the might of Kil'jaeden!

Not confirmed per-se, but a room named Frostmourne has indeed been datamined: YouTube - [spoiler] Icecrown Citadel - Frostmourne Area