Syl wants to know what makes a boss fight memorable.
I was the only person to have anything close to a scientific answer based on my extensive research in the growing field of "things I sort of remember". In other words, I am an expert Moltencoreologist.
So the answer: Tall bosses. Big rooms.
These are both essential. Short bosses in very big rooms don't work. Those are called gnomes and they are utterly unmemorable except if you kill thousands at a time.
Conversely, big bosses in small rooms don't work. Note that this is relative. While Magtheridon's room is big and he is big, he is too big for the room.
A cluttered room can simulate the effects of a small room. For example, Kel'Thuzad's room is filled up with a giant throne and the walls are filled with hostile undead, reducing the effective space. Furthermore, his small footprint means that melee are packed up, reducing the apparent size for them. That other KT, Kael'thas, has rooms scattered with banners, statues, and strange glowing boxes, which also reduce the perceived size. This is in addition to him ultimately dying in what appeared to be a large storage closet.
We've seen all the negative examples. What about fights that worked?
Raganaros tops the list. He is very tall. And wide. The room he's in is gigantic. The size of the room has the benefit of allowing players to stand far enough back to see his head.
By this point you may be wondering, is this just another silly post? Yes. No. Sort of. I'm trying to find something universal here. Something applicable. Challenging fights are dependent on the skill of the players, their gear, and not getting nerfed two months later. And they can't be replicated. Once you know a fight, you know a fight. Challenge tends to be a brick wall which is suddenly gone, without the slightest hint that it was ever there. It cannot instill feelings if "you weren't there then". Silly fights, such as the pygmy in Lost City, don't retain their value. The joke gets old. But maybe a tall boss in a big room has some lasting power. It is not a brilliant design or a substitute for a fun, challenging fight, but as a general rule, the last boss should be tall and in a big room. A well-decorated room.