First, let me say that this analysis is based on fluff rather than TT game terms. In-game, I obviously understand the decisions made in order to preserve game balance. In fluff, though, the Craftworld Eldar could be a giant economy-size tub full of kick-ass.
#1 - Tech. Their weaponry is as advanced as anyone; they have light troops with decent personal armor; their vehicles are light but as mobile as anything; their specialist troops have some real kick-ass gear; their fleet is decent; they have a small number of infantry that can actually jump around the battlefield through the warp; and holofields and D-cannons.
#2 - Weird brains. Not only do they have powerful psykers as a matter of course, they have powerful psykers both geared toward combat, and others who can radically alter the course of events on the battlfield.
#3 - Hordes of the undead. The Eldar could field huge numbers of wraithguard and wraithlords.
#4 - Specialists. The Aspect Warriors mean that troops specialized in just the right type of combat for a given situation are available. Add to that the Rangers, the Autarchs, and even a friggin' Avatar - which is just what it sounds like, a war-god reborn in flesh - and you have some pretty formidable situational solutions.
So, with all this, why are the Craftworld Eldar a bunch of flighty fruits instead of gum-chewing ass-kickers who just ran out of gum?
Segmentation. No one craftworld has the numbers - of bodies, of armor, or of each different type of specialist - to make any true headway on their own. Unfortunately for them, each craftworld is too fixated on their own particular goals or biases to overcome this with cooperation... to say nothing of each one's prejudice against the others. One may be intent on killing orks, another on reclaiming maiden worlds, another on fighting Chaos, and each one things the other is wrong a/o possibly tainted. In fact, one's bonesingers would complement another's Aspect Warriors, which would both complement a third's Rangers, and another's warlocks and farseers, etc., etc.
The Astartes wouldn't send just assault squads and landspeeders to try to hold a position - but the craftworlds are left in that situation. One craftworld may have Dark Reapers, for example, and no Swooping Hawks or Warp Spiders - but may still have to try and fight a guerilla war on their own nonetheless. If that craftworld were, instead, working cooperatively with another that had such Aspects but no heavy support, each would gain from the resources of the other and together they'd be able to field a more flexible and situationally-appropriate force.
In other words, if they ever just got over themselves they'd be pretty tough S.O.B.'s. Figure out what you want as a species and then figure out how to get together and get shit done. Oh, you have scruples against over-use of dead warriors' spirits to create vast armies of wraithguards? Well, tough titties - you're at war, get over it.
"You fought with Captain Reynolds in the war?"
"I fought with a lot of people in the war."
"And your husband?"
"I fight with him sometimes, too."