Well, with every codex there tends to be a slight retelling of old fluff, and the Eldar are no different. For me, anyway, the background section of this Codex a major disappointment. Not only is the rumoured expansion on the War in Heaven not something that apparently exists, but the majority of the events of the 13th Black Crusade are notably absent. In particular, it seems Eldrad is alive and well, doing his mystic mumbo-jumbo.
The timeline is, in a word, crap. Half the events of other armies, and only two of them really bring home any of the "Eldar PWN" vibe you would expect. Really, the timeline feels more like Sisters of battle (you know, bunch of draws or narrow victories) than anything.
Worse, they seem to be really beating this 'End Times' horse with as many sticks as they can. 40k has always had far more than its share of doom-and-gloom, but this is getting old, really fast.
I have noticed a few interesting inclusions, however. First being the inclusion of some hint towards the nature of the much-implied Chaos Taint on the people of Alansar. They seem to be either unwilling, or incapable of removing their armour, at least in the presence of outsiders. This could allude to some rampant mutations, or perhaps they have fallen prey to the same type of ill-fated ritual which created (and in a way destroyed) the Thousand Sons.
Then you have the information, under Asurmen, that some Warrior Paths predate the fall. In Asurmen's snippet (i like having more than a sentence about our Phoenix Lords again) it says he abandoned the Empire before its collapse, and that some of his students returned to found their shrines as the Craftworlds were fleeing .
Also rather neat are the Wraithblades, who are powered by Spirits who couldn't let go of death and bloodshed. They seem treated in a very similar way to the Hellbeasts of Chaos, separated from the 'normals' and only used under the strictest of controls or the most desperate of needs.
I also found it rather interesting/funny that Illic's rifle contains the spirit of the Ranger who created the Remnant of Glory sniper rifle.
All in all though, the background section kinda sucks. Nothing really added (beyond descriptions for new units, of course) and we actually lost some background.