NOTE: This list is just meant for entertainment purposes and people who might find it interesting. I wrote it in quite a short time with only using my own memories and my own opionion after being a Hardcore/Semi Hardcore raider since 2005. So if there is a mistake somewhere or someone thinks he knows better, then you may of course discuss about it but please no "xy isnt true you are so f****** stupid" (bad enough that i have to write something like that).
T1 (Onyxia, MC, World Bosses): Tank & Spank. It was the Core Element in this Tier therefore it was vital to not let the Tanks die so take as much healers as possible. To some degree it also introduced Hunter Pulls which was even more important in T2.
T2 (BWL): Line of Sight, dispell everything, Aggo and Fear. The usage of line of sight was introduced and was a Core mechanic for most of the bosses in BWL. 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), Chromaggus still is THE dispell (and line of sight) Boss till today 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.
T2,5 (AQ40): Enrage, Positioning and Magic-Resistance. For the first time it was actually important what your Damage Dealer did. While you could heal dead all bosses before some bosses now got a lot easier if you had good DPS (Fankriss), had tight enrages (Huhuran) or a berserk (Twins). Cthun also was a DPS Check if you didnt want to get overrun by tentacles. Also it was now a key element for all raiders to farm Resistance Gear while it before was mainly a thing for Tanks. Cthun P1 introduced the need to make exact positions for every raidmember but other bosses needed fix positions too.
T3 (Naxx40): Individual Responsibility and Gearchecks. Basically for the first time all raiders had to prepare Bossfights carefully because if you didnt know what to do in certain situations you would die and maybe even kill the Raid. Best examples are Thaddius, Heigan and Sapphiron where you would simply die if you didnt run with polarity, didnt know the Heigan Dance or didnt run behind the Ice Blocks. Moreover some bosses had Voidzones as big elements of the fight meaning if you didnt react you died. With Patchwerk you had the first pure gearcheck, if you didnt have enough DPS, HPS or Tanks without enough gear you would have no chance.
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 also had the first real Raid-CD with Bloodlust.
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. Not that there havent been adds before, but it was a core element of the instances.
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.
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...
T7 (Naxx25, Malygos, Sat): 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.
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. Mimiron HC is still a reference for all those "Mimiron 2.0"-Fights
T9 (TotC): Limited Attempts. Explains itself doesnt it? Used in ICC too but then abaddoned.
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!).
T11 (BWD, BoT): Healingzones, Raid-CDs and DMG/Heal Hybrids. Every Healer now had his "Healingzone" where you had to stand in for effective healing, so from this point on stacking your raid always meant a lot more healing. Raid CDs were present in Wotlk too, but now a lot more classes got them and it was way more important to properly use them (Aura MAstery, Tranq, Spirit link, etc). Disc Priests being able to heal by dealing DMG is a design that has a big impact on bossfights until today.
T12 (Firelands): Basically everything that was used on Ragnaros HC. Additional HC Phases, "Kite the Boss into xy", Adds that arent allowed to reach a destination, World in Flames-Mechanic, etc.
T13 (Dragonsoul): Extra Action Button, Voidzones you have to stand in and choose how the fight goes. Our dear friend Extra Action Button had its first appearance. Voidzone you have to soak werent completely knew i think but now really important until today. Yorsahj and Madness introduced ways you can change what mechanics bosses use (choosing colors and what plattform to start on Madwing), what will still have in MoP like Protectors or Siegecrafter.
T14 (MV, HoF, ToeS): "Throw the Ball" on Sha of Fear HC.
T15 (ToT): Healing-Absorb-Fights like Tortos. Dark Animus was kinda unique Encounter but dont know how to call his "mechanic", pretty sure we will see something like that again.
T16 (SoO): "Minigames" in Bossfights. Solo-Proving Ground at Norushen, Pacman at Sha of Pride, Assembly Line at Siegecrafter. Nothing gamechanging but i'm sure we will see more of this in the future.
Pretty sure i forgot some things but in the end these are only elements that still have effects on raiding nowadays or at least had it for a long time.