I am a huuuuuge fan of Aion's lore and everything about it. I can't tell if I've completed every single quest, mostly because I skip the collect/group ones, but I did read every single quest I have done on both sides, every single book in the game and most of what can be found online.
But you know what is probably the most interesting thing to read in game for me ? NPC default dialogues in the first cities, especially Sanctum/Pandaemonium. No one cares about it, yet it is full of precious informations about the area, the relations between everyone and the lore itself. Even the most insignificant guard standing at the entrance of the capital has a little gossip to share.
A lot of people underestimate Aion's lore in my opinion, as it is not directly told to the player. It needs to be searched.
And maybe because it can be fastidious to read everything as the game gives lots of quests at the same time, I can understand why most of the players give up on it.
Now when I see the current official Aion and what's left of the lore ... hmmf, no comments.
Another thing I regret about what they did to Aion, is the change made to the sky and the Tower of eternity. It is the most important part of Atreia, yet it was totally scrapped from the game. For those who forgot about it, here's an old screenshot. And the "new" one.