Classic:
T2 (BWL):
Line of Sight, dispell everything, Aggo and Fear. Fear required your Tanks to learn Stance dance (i know Magmadar also had fear but could also be killed with feared tanks while Nefarian would wipe you
fear was introduced in Onyxia fight, when feared tank would be dead raid due to fire breath), Chromaggus still is THE dispell (and line of sight) Boss till today (
You had to dispell in MC) and healers had to adjust their interface to see debuffs or get decursive. Moreover you always had to watch for your aggro from this point until end of wotlk or something like that because most bosses were not tauntable and had aggroreduces on tanks (
Overaggroing on Onyxia would HURT the raid real bad).
T3 (AQ40): Different kill order gives different abilities to bosses (correct me if I'm wrong)
T3 (AQ20): The AQ20 brought more new mechanics. The 'bring the boss near eggs then kill eggs to damage boss', the 'one person dies if boss reaches him', the 'this massive area is for boss fight', the 'drain mana from boss or you wipe'
T3 (Zul'Gurub): everything mixed up with the 5man remake >.< and a bunch of memory holes since I can hardly remember the old ZG
TBC:
T4 (Karazhan, Gruul, Outdoor):
Not only Warriors are Tanks, Bloodlust and spreading the Raid. It was just indirectly because of the content but for the first time Feral Druids were really viable tanks, we also got the first encounter where Ranged Classes could tank Bosses! Gruul was about spreading out properly (
you had to do that in MC). You also had the first real Raid-CD with Bloodlust (
how does raid CD qualify as new raid mechanic?).
You seem to have forgotten Karazhan, chess event, opera event (randomized boss), Shade of Aran - 'DO NOT MOVE', Netherspite - had to manage 4 beams and rotate people for them. Prince - whole raid goes to 1hp?
T5 (SSC, TK):
You are the bomb! and handling Adds. While we already had "Bomb" Encounters before like Geddon Solarian brought it to the next level. Pretty much all fights had adds and because the Trash was so hard you had to learn to CC and handle all kinds of Adds (
only 4 bosses in MC didn't have adds). Not that there havent been adds before, but it was a core element of the instances (
Thats what made TBC dungeons hard, imho).
SSC first boss - had to drag him through beams, never seen before. Fight in 'water', also having to fish for boss.
T6 (MH, BT):
Dont stand in the Fire! There were some more mechanics and its not the first time with ground effects. But for example Archimonde was THE "Dont stand in the Fire" Boss. (
Prince in Kara.)
BT: Reliquary of Souls. Also, bringing Illidan into a trap (I think nothing like that was done before)
Wasn't T6 when NPC's started helping us in Fights?
T6,5 (Sunwell):
Groupheals. While Chainheal was important before being able to effectively Groupheal was more important than ever and you wanted to stack up as much as possible to outheal Raid-DMG. DPS was also very important but this was no really new mechanic... (
going into portal to fight another boss was introduced in SWP. Also turning into a dragon and using skills? I guess Naxx did it first with proffesor, though.)
Wotlk:
T7 (Malygos, Sart):
Choose your difficulty and Achievements. Sartharion was the first boss with a Hardmode. The content was also designed with Achievements being the "competitive content" but it failed miserably (
In what way did achievements failed? I think they served their intended purpose: look at my E-peen everyone).
Maly: wasn't it the first fight 'run into the bubble or you die' type? I believe it was the first fight to have flying targets?
T8 (Ulduar):
Hardmodes, Secondary Resource Bar, Mimiron and Vehicle Fights. The first Instance designed around Hardmodes, for Yogg-Saron you had an additional Resource Bar (Insanity) which was often used again in other raids. Also the first and only pure vehicle Fight (
Chess Event in Kara). Mimiron HC is still a reference for all those "Mimiron 2.0"-Fights
(
never heard that reference before, can you list them?)
More in Ulduar: Kologarn (only similar 'remake' was Deathwing, to a lesser extent.)
T9 (TotC):
Limited Attempts. Explains itself doesnt it? Used in ICC too but then abaddoned.
Not sure how that qualifies as mechanic?
T10 (ICC):
Healer-Boss and unrecoverable deaths. Valithria was the first Boss that had to be healed full instead of killed, also an element often used again in other raids. As it wasnt bad enough if you died in a voidzone or due to a mechanic, you could now also die without being Rezz-able (Lich King!).
MoP:
T14 (Heart of Fear): Garalon and that boss where you have to run against wind.
T15 (ToT):
Healing-Absorb-Fights like Tortos (I don't know what you mean here). Dark Animus was kinda unique Encounter but dont know how to call his "mechanic", pretty sure we will see something like that again. (
I would compare Animus fight slightly to Omnotron system)
T16 (SoO):
"Minigames" in Bossfights. Pacman at Sha of Pride (
is that on normal/hc only? if not then I don't know what you mean).