Fey and dangerous. I'm a huge not-fan of the D&D uber-organized frightfully-lawful city-dwelling elves that you find in Evermeet, for instance, or scattered around the Dragonlance setting.
I prefer them to live 'under the hill' and be masters of illusion and deception, being otherworldly in nature and temperament. Totally chaotic, and beset with unnaturally strong emotional swings, an elf could be your best friend, when he's drinking with you, and then be swinging his sword at your head after an ill-considered joke.
This sort of elf is the definition of 'fey,' in the sense of all-but unknowable to the mortal races, with no guilt, no shame, and never dwelling on what they did yesterday. They're like two-legged sharks, creatures of sometimes violent jaded passion. This entire mortal world *is not their home* and none of the creatures in it, ultimately, seem more substantial than mist and sunbeams to them. Killing a man in a flash of temper, or setting fire to a town just to watch the little mortals run around screaming, is a meaningless act to a creature from another world who is probably going to outlive that man's great-great-grandchildren, and live long enough to see that town abandoned anyway.
None of this 'one with nature' crap. They are creatures of glamor, and, to them, this entire world is just another 'illusion,' like a canvas that they can paint on, or not, as their whims take them. They aren't from this world, and they could give a rat's butt about living in harmony with nature. It's all going to die anyway, what does it matter if it dies a few days, decades or centuries ahead of schedule?
Elven heroes accomplish amazing feats, often times against seemingly impossible odds, *because they don't care.* They'll take any risk. Life is meaningless to them, and the longer they trudge around the 'mortal mud world' the less they fear death, and the more they begin to obsess about 'the next world.' Unfortunately, for any inspired to accompany an elf on such a 'daring' mission, he cares even less if *they* survive the campaign either...
For all that, elven villains are extremely rare. They simply don't have the attention span necessary to commit to a campaign of evil, spending the majority of their lives wandering from place to place, doing whatever strikes their fancy, in an attempt to stave off the ennui inspired by their centuries of life. A suprising number of this rare people loathe nothing so much as the company of their own kind. They don't appreciate the irony of how terribly annoying their own jaded sensualistic nihilistic worldview looks when they end up in the same place together, and quickly end up holding each other (and themselves) in contempt.