Mechanically, I feel like Blizzard has fairly perfected their formula for raid design. Not necessarily that all the raids are perfect, but I haven't really heard any resoundingly negative comments on a raid since Dragon Soul. Since Dragon Soul, they've worked on creating larger raids with more unique bosses throughout MoP, then worked on reworking raid size/difficulty and making equally large but less linear raids in WoD. I feel like the progress they've made in MoP and WoD mostly satisfy all of the mechanical/gameplay requirements of a good raid, and as long as they keep that up, then, in my opinion, the biggest deciding factor of the memorability of future raids will be their art/layout design, lore, music, etc.