The Changeling topic also got me to thinking about this: how are we defining the fae in Serendipity? I think it's important to have a clear vision because this will impact Serendipity. ^^
For me, fae are creatures that are neither good nor bad. Some (like the seelie) are more inclined to do good while others (unseelie) are more inclined to do bad, but they're still neither wholly good nor wholly evil. They are as a whole flighty and mischievous, but the difference is that seelie are more prone to be beneficial and helpful; they warn when an accidental offense is committed, they aid people, they ask for help, and they choose to help before they harm. However, they are also selfish, easily bored, and seek to avenge insults, and their version of "helping" may not always be what one was looking for. xD If angry enough, they may outright hurt people in vengeance.
Unseelie, on the other hand, will choose to harm before they help, if they have a choice between the two. They don't really care much for humans, and when they do cause harm, it's downright malicious--such as what púca/pooka do. They're not entirely bad and malicious, though, and may help at a price. But mostly, they enjoy inconveniencing people, assaulting them, and driving them off.
Both types may be responsible for changelings, either as pranks or acts of vengeance, though unseelie are more likely to make the swap.
My personal interpretation! xD And I like the idea that Serenians could be descended from either.
But yeah! Thoughts?