I've been pounding my head against a wall trying to come up with a good 6th edition Eldar list. It's hard. It was already pretty hard in 5th. Eldar are overpriced by at least 20%, the good units all fight for the same FOC slots, and 80% of the rules are unchanged from 1999. While 6th gave a boost to certain aspects of the army, it also hurt some others pretty badly.
You already had to squeeze every ounce of efficiency out of the codex to stay competitive in 5th. We could argue the validity of footdar versus mechdar, but either way many of the units in the codex where still untouchable. Barring Firedragons, most everything else in the codex was situationally good at best. There where a couple of nice gimmicks you could run like Eldrad+Phoenix Lord+Wraithgaurd. Unfortunately these could be trumped in competitive environments, and they are expensive points sinks.
The changes to how units in transports score(or not score more appropriately) and wound allocation have gutted a lot of the tactics that allowed Eldar to stay somewhat competitive. Eldar just can't grab and hold objectives like they used to. You have to get out and try to survive, and if you payed the points for a Wave Serpent, you certainly didn't have the points to spend on putting a survivable unit inside. I also found at least a dozen rules that make no sense or are redundant/pointless/useless/dumb in 6th(though Tau have it worse in that area).
There are still some gems. Karandaras and a few of the other Phoenix Lords got a nice boost. Wave Serpents weathered the addition of hull points better than some other vehicles. They may even be over-costed by only 15% now. Twin-linked strength 6 is about as good as you can get for anti-air in a codex without it's own fliers or sky fire. Now that Warp Spiders can tear up vehicles, Eldar have a fast attack unit worth taking. Guardian Jetbikes are nice too, but I can't brink myself to buy the old 2nd ED models for 15 bucks a pop when I know that there's a master already done for a new kit(Jes Goodwin redesigned the Eldar jetbike as a precursor to re-designing the Dark Eldar ones). And with GW's black hole of a marketing strategy, a new kit could be as little as two weeks out and I wouldn't know it.
There's also some changes that look like buffs, but turn out to not be so useful in the long run. The Psychic power change is one of them. Eldar need fortune, and the analog to it in the divination table only effects the psyker and not the squad. So you need a cheepo Farseer to cast Fortune, and another one to roll up the fancy new powers. Now you're locked out of taking anymore HQ choices; one of the slots in the codex that has an abundance of useful choices.
You may also think that Ranger/Pathfinders got a huge boost with the focus fire and cover changes. They are better, but not better enough to account for their 19/24 point cost. You can't afford to invest that many points into a unit in an Elder unit unless it's going to be able to wreck face, and these guys will not wreck any face. You'll also love it when some 6 point guard model with a flamer walks up and makes your 240 point investment disappear in one shot.
Allies could potentially save Eldar, but allies tend to put an efficiency hit on the army that takes them. And allies are hard to make work at the lower points levels. 2 FOC might help too, but you usually run out of points before you can get enough spam into the list to make it worth it. Plus double FOC is dumb.
Eldar need a new codex in a bad way. I know they're xenos, and xenos armies are never quite as good as Imperial. I don't expect GK or SW good. I just want to at least get within visual range of competitive. Tau are't great, but I can make them work with good play and an efficient list. Most of the Tau codex entries can be made to work. They only have a few absolute stinkers(Vespids, Etherial). Oh, if Eldar only had it so good.