I haven't read up on the Eldar all too much, so I'm left with a question: Are the Eldar actually on a completely different level, mentally... or is that just mankind's perception of them? And does anybody have any idea how the Dark Eldar are going to be even as playable as Orks and Kroots? Or what would and Eldar game be about? Would we play some group of Eldar who hunt down Chaos worshippers and Warp artifacts to store inthe Black Library or somewhere similar? And am I recalling correctly that it is now forbidden for Eldar to perfect any art out of fear of the whole "we created Slaanesh" thing? So no Eldar may stick to 1 profession forever? So they'd have to "multiclass?" I just don't know. Woof, more questions than I thought.
Some canon fiction portray the Eldar as DEEPLY alien and inscrutable, others less so. They are as playable as pretty much any other fantasy or alien race in any other roleplaying game.
I think that the wise idea would be do start out with a "vanilla" Eldar core book, potentially with classes like Warlock, Aspect Warriors, Pirate, etc. The smart focus would be on Outcast - jack-of-all-trades that have a reason to wander about and interact with aliens and the Imperium. You could try a Dark Eldar supp, that that has the limitations of a Black Crusade game - all the players are Dark Eldar, or none are.
The game could be about *anyting* - exploring Maiden/Crone worlds, trying to "understand" the Imperium, thwarting Chaos, becoming a Pirate, being an Eldar in a RT retinue, etc, etc. I believe that the Eldar switch professions several times over their long lives simply because their lives are LONG. As far as multi-class, you could handle it like DH - work up one class/tree, then stop and work another. Aside from Thorpe's really bad Eldar books (IMO), I think that you wouldn't have to do this much - the Eldar seem to focus on professions for decades/centuries, not over a few adventures or years.
I would think that an Eldar theme would support a core book, and then supps dealing with Aspect Warriors, Advanced Classes, Pirates/Ships, Craftworlds, Maiden Worlds, your obligatory adventure trilogy or two, etc. I think that it is a real missed opportunity.