1) First night of BC, still in my old gear, grouping with some top players on the server (as a somewhat no-name tank), and clearing both Ramps and BF. All of us were 60, which made BF quite an accomplishment.
2) Getting my First Primal Nether, and crafting Tankatronic Goggles on the spot. It was the Demon boss in Ramparts, and my first heroic. The rest of the party knew I needed it to make the goggles, but were probably somewhat surprised when it all of a sudden poped into chat that I received them, and that moments later on inspecting me, it was already glyphed and gemmed!
3) Getting the Gruul Shield. This was my first Gruul run with LOKI. The funny story behind it, was that a DPS warrior had more DKP then me, but someone else on vent suggested he pass it to "Yak". He agreed, and I got it (Whoo huge upgrade!). It wasn't for months after, that I found out he thought the person was talking about a different tank, Yacoolwoman, whom went by "Yac".
4) Becoming LOKI's Tanking Officer. It was about as big an honor as I thought I could get, both in terms of recognition of my abilities, and leadership qualities.
5) MTing / Killing Vashj. This major victory marked the start of LOKI's rampage through end bosses. It also solidly put us past Icon, a guild whom I had considered a rival at the time (They were on Vashj when we started SSC, and didn't down her till we were working on Kael).
6) MTing / Killing Kael. This happened a few days after attunements were lifted from BT and MH. LOKI hadn't set foot in them, as we were focused on finishing one tier before continuing. To top off the achievement - I got his orb-quest-turnin item. We collapsed as a guild into Shatrath for me to turn it in.
7) First 4-Chest ZA Run. It was probably the best continuous forty (?) minutes of playing I've ever seen, from myself and others. And then we all sat and laughed at the crazy gnome while we waited for the bear.
8) MTing / Killing Archimonde. For the last 10%, I was just so relieved about not having to worry about fears. I could hear the shouting all around me of "Wooookillhimkillhimkillhim", but I was mostly relieved to be done with my part.
9) MTing / Killing Illidan. After he owned me in ~1.5 seconds following the pre P5 bubble, It became a personal vendetta. Shears never bothered me as much as Archimonde fears, though.
10) Becomming GM, and having LOKI not miss a beat. This was both a huge honor, and a very sad moment. Watching Ruff leave was tough, but having zero gquits and a followup Illidan kill the week he left / our first kill, was a huge for a guild. I knew I had the support of our members still.
11) 5.5 Manning Kara. This was the most fun I had in Kara, going at it with six people (post 3.0), including a very fresh 69 Hunter, and (previously) PvP only Paladin. I didn't think we'd be able to get through it (Netherspite being a fun stretch), but we did, and it was more fun then I had had in there, in a long time.
12) Seeing Kil'jaden, Pre-Wrath. Realistically, with last night's extended-extended-extended server maintenance killing half of the remaining potential raid nights before the expansion, we have at most one night in Sunwell. Enough to clear to him, but probably not enough to put in any more serious time on him. I did, however, get to watch him emerge, yell at us, and go through three phases - at the level intended.