Several come to mind. I remember working on Lucifron in MC for several weeks before getting that first kill. With "raiding" being UBRS/Scholo previous to that, the Core was amazing. Getting KT and Vashj, Archimonde in Hyjal, LK in ICC. Like others have said, there doesn't seem to be the level of time and effort working on encounters before finally getting that first kill. Hence it takes away from the excitement and energy when it does finally happen.