If I ever got to playing with my idea of elves...
A lot of what follows is clearly drawn from Exalted and Dark Ages Fae. Parts of it (particularly the bit about bodies) have something in common with Starman (the film with Jeff Bridges as an alien).
An idea I've always wanted to play with is having elves be the "clothes" of creatures that originate beyond what is known as reality. Their ultimate origins are unknown. They could have come from the chaos of pure potential. They could be the dreams of even more ancient beings, or something else altogether. For simplicity, I'll call them fey.
When the fey became aware of the mortal world, they were intrigued. Many were curious and wanted to learn more about this world. Some were filled with the urge to nurture and protect them from dangers they were blissfully unware of. Some wanted to order this world to their liking and have its inhabitants as their subjects. A few wanted to do all these things.
Their fascination with the world and creatures in the mortal world led them to devise bodies which would make it possible for them to interact with it. Based upon their (incomplete) understanding, those that wished to enter the mortal world set about making bodies for themselves much in the same way someone going on a long journey makes clothes suitable for the trip.
Creating bodies was a trial-and-error process. Making exact duplicates led to confusion ("Is that you, Earl? I thought you were dead!" "I didn't know Bob had a twin brother."). Winging it led to the creation of bodies that were almost completely inhuman. In form fey could seem like angels, demons, and monsters, though their nature was often different. Eventually, most fey got the main details of the humanoid form right: one head with two eyes, two ears, nose, and mouth arranged in a humanlike way; two arms with hands, fingers, and opposable thumbs (though number is sometimes a problem); two legs with feet (toes optional); and a humanlike torso attatched to said arms, legs, and head. Depending upon their understanding of things, a body may or may not have accurate internal organs, bodily fluids, and genitalia.
Yet, even the most experienced and observant fey couldn't get everything right. Their features were often too perfect, precisely matching the major geometrical shapes which make up the human form. Their skin had no blemishes. Eyes would have slit pupils or strangely colored irises. Hair would be absent or colored in a strange way. Complexions far beyond the human range. The wrong number of fingers and toes (usually six or four digits on each hand or foot). Ears that are slightly pointed. And these are the mild cases, which could be mistaken for deformities. More subtly, mortals who watched these creatures long enough would notice that they never aged nor became ill.
As the trend caught on, fey started forming their bodies not merely for blending in, but for self-expression. Through these bodies, they realized that they could achieve their goals in the mortal world.
The adoption of physical forms was not without effect. Though neither age nor disease could afflict them, the fey were not impervious to pain or injury. They also found that their bodies limited their powers tremendously. Even the most trivial of things required a great amount of effort on their part. One theory is that their origins in a time and realm of near-infinite malleability was at odds against the more fixed reality of the mortal world. By going into the mortal world, they more or less had to abide by mortal rules. However, with time and experience, they could regain some of what had been lost in the transition.
As fey interacted with mortals, differences soon became apparent. First of all is the inevitability of death for mortals. Death has perplexed them since it first became known to them. The body suddenly stops working despite the lack of illness or injury. The mind and spirit depart to a mysterious place. Or does it simply disappear? Death has been a source of great interest, frustration, and heartbreak for the fey. Some have taken to performing experiments in the hopes of untangling the mystery of death. In other words, they kill people and try to see what happens.
Second, and probably most important for their aims of interacting with the mortal world, is a thing mortals refer to as morality. The notion of good and evil is completely beyond the fey. They can certainly understand the consequences of their actions. They are sometimes capable of guilt and shame. Yet these things are rooted not in abstract principles but in how it affects them and those things and people they are attached to. Benevolence and malevolence are within their nature, but they generally lack empathy. They cannot take the figurative walk in another's shoes. Even if directly asked what they would do if they put themselves in the position they seem fond of placing others in, they frankly would not understand.
At times, they can develop empathy and can learn to understand and appreciate mortals for what they are and not for what they can do (or do for them). It is certainly very rare, but not impossible. Sometimes they even find it against their best judgement. However, even as it makes them more vulnerable, it also enriches their experience of the mortal world.
Third is the basic psychological makeup of the fey. Compared to mortals, fey are simple creatures, but what they lack in complexity they make up for in power. Fey are creatures of unadulterated passions. They can change quickly, but they are always pure. They are not infatuated; they are obsessed. They don't dislike; they hate. They don't feel hope; they have unshakable conviction. They don't feel blue; they despair. They don't like; they love. Some of them have learned to disguise this aspect of themselves, but you'd better believe it's there.
Finally, there is the impact of the geas. The origins of the geas are unknown. Some believe that the first fey who created bodies for themselves created the geas out of an attempt to mimic human morality. Some think that it's simply a condition of assuming physical form. Some believe that the geas is imposed by other powers. Regardless of what one thinks or believes, the practicalities are the same. The geas is a magical compulsion or prohibition which a fey must follow or risk a terrible fate. The form of the geas and the penalties for breaking it are different for each fey, but there is always a sense of bitter irony when they suffer the repercussions of breaking it. Neither the geas nor the curse can be avoided or altered in any way short of divine intervention (which has not happened yet).
Whatever culture or society these creatures have is modeled from what they experienced in the mortal world. Their reasons for gathering in numbers varies, although shared goals is the most common. They often use secrecy as protection, whether it is by carefully hiding within mortal society or isolating themselves from it. The places where they live or gather tend to be known as magical places where time and place become more fluid. Decades can pass by in a single night. What seems like days is only a few minutes. Whether these places are helpful or hostile depends upon the temperament and motives of the creatures gathered there. Those who take their inspiration from feudal societies may have a faerie court of some kind. Those who prefer democracy may establish realms with equal opportunity for carnage and destruction. There are other models as well. Mortals are advised to take nothing for granted, for something is rarely as it seems.