Achievements of the group
The randomness of the encounter is the bit they got right. After all, how many people have downed Algalon? I wonder how much of these percentages are random chance or whether they are some how tied to the Achievements of the group? My common sense tells me the chances of getting Onyxia over, say, Illidan is more than likely tied to the kind of boss, the level/complexity of the content and when it was released.
The other aspect of this is whose memory Pale tress is summoning. After all, we all started at different times during the lifecycle of aion gold and most of us have pugged certain content. What happens if you get the Memory of Vashj, for example, but never ventured into Serpent shrine Cavern or Archimonde but never went into Hyjal. Yes the ability to play against different bosses does make the encounter an interesting one, even if this is only on a physical level, but raids and bosses need more than that. It would be nice.
