Unfortunately for me, Blizz killed each and every fun instance I ever enjoyed with Cata. VC? Changed. ZG? Butchered. And I actually solo'ed to 60 so I never got to do any of the end game raids in Vanilla.
I'm confused: ZG was one of the fun instances you enjoyed, but you never got to do it? O.o
Also, people used to do things like that with Geddon's bomb and (more famously, perhaps) the blood plague from ZG on purpose, too. It's why they made that change. ;p
As for memories...I remember the huge breakthrough moment in raiding that happened when they introduced the Sunwell in BC. Not because of the new raid/instance content - that would be later - but because of the new gear vendors they added for massive upgrades off of heroic tokens. Overnight I went from a middle-of-the-pack affliction warlock to a decked-out 0/21/40 boomlock as I'd had a disgusting surplus of heroics done and could afford to get literally everything worth getting in one shot.
The best part? No one else in my (newly-joined - I'd been with them maybe two weeks before this) raid team understood the ramifications of the new gear being available, nor did they know until our next raid night that I'd completely changed spec and gear.
After the first boss in SSC exploded
and raid lead checked the meter to see that I was something like 20% of the raid's entire damage dealt by myself, there was a hearty round of wtf
. And thus, a legacy was begun.