All bosses in a raid heavily tied to lore.

[Reyzzz]

Let me be Exhibit A. I had no idea that anyone but Yog'Saron and Thorim were known about before walking into the raid.

Freya was present in sholazar basin. Ymir and mimiron were brought up in storm peaks quests.

[Chonar]

I, like others, think this would be rather over the top. As long as bosses don't seem totally out of place and/or in too important/powerful a position without us having at least heard of them before (I'm looking at you, Ultraxion and Warmaster Blackhorn), they don't need a huge backstory.

Like Lei Shi, for instance. We know there's elemental sprites in Pandaria. We know this is one. We know it's powerful from playing in the Terrace's pools. We know the general attitude of the Elemental Sprites. That's enough.

Also, consider Gara'jal the Spiritbinder. We never heard of this troll before. But in Mogu'shan Vaults he says he will be back... and guess what! He will, as an encounter mechanic in the next raid.

That's lore being written as we experience it. That's great. I actually would say I prefer it over a long and intricate backstory that has little to influence on/relevance to the encounter.

[LazarusLong]

This is one reason why I like ICC.

Many of those bosses have some backstory behind them, some even a great backstory.
And even those without a backstory fullfill a role that fits into the lore (e.g. Putricide).

Tirion, Bolvar, Airship battle (Muradin), Deathbringer Saurfang, Blood Prince Council. Lana'Thel, Sindragosa and of course the Lich King himself had at least a good implementation in lore, if not a major one.

To me, ICC was a kind of "WoW All Stars".

Really? And who was Lana'Thel? Profeccor Putricide?

What the hell did green dragon do inside ICC?

I hope in 5.2 they will expand hints like in Klaxxi dailes (Not just bombing run or grand finale) so you can actually interact with raid bosses before you step into raid.

[Chonar]

Really? And who was Lana'Thel? Profeccor Putricide?

What the hell did green dragon do inside ICC?

I hope in 5.2 they will expand hints like in Klaxxi dailes (Not just bombing run or grand finale) so you can actually interact with raid bosses before you step into raid.

To be honest, you wanted to have done the Quel'delar questline to properly get introduced to Lana'Thel.
Not everyone got to see that. (But it was awesome!)

[Vyrd]

first of all to resolve the whole ulduar confusion.
yogg saron, the keepers, loken, and razorscale were all heavily referenced, used and interacted with in storm peaks.
if you complete brann's quest chain along with the earthen and the sons of hodir, you learn all about the keepers, loken, thorim and yogg saron.

now to the mega lore raid that could come to be.
first things first, demons do not 'die'. their body is destroyed, their soul returns to the twisting nether until they can re-manifest themselves.
i only see really one possible way that this could be done. in an end of burning legion story raid. kil'jaeden, gul'dan and teron gorefiend. the dreadlords; varimathras, detheroc, anetheron, mephistroph, balnazzar, tichondrius and MAL'GANIS (his story was fucking DROPPED HARD after completing that quest chain in icecrown, i was mad). the pit lords mannoroth, azgalor and magtheridon. azzinoth and houndmaster hakkar (not troll god hakkar), kazzak and kruul. those are just a handful of the known burning legion lore figures. fortunately mostly all of them can be used more than once. demons do not die.

if blizzard goes with the immortal demon story, illidan is bound to return as well, seeing as how he was a demon. target him in game. he is level ?? demon.

[Chonar]

Vyrd; There is no effing way Blizzard is gonna put Hakkar the Houndmaster in the game. People unaware of the naming mixup (which Blizzard admitted was their fault for poor communication between the game's lorekeepers and Knaak) would be so confused.

The Houndmaster will probably end up being renamed. Harrak the Houndmaster or something.