I've been told my Elves aren't Elvish enough before for a variety of reasons. (The time between Feanor's children in Felak!verse being just one very minor example of it!) When you consider the wealth of Elven characters and their varying personalities and responses to conflict, though--as you point out--"Elvishness" becomes harder to define.

I would have a hard to criticizing anyone's characters for not being Elvish enough. I can imagine that, shortly after Tolkien's death before The Sil was published, it slipping to a Tolkien community in the form of a fanfic and people immediately leaping on it and saying, "Feanor isn't Elvish enough! No Elf would make such hasty, stupid decisions and then turn around and do such a heinous thing as the Kinslaying! I mean, we thought Thranduil was evol! I think you need to label this AU."

Jenni's right in that there's just so much information to choose from. And I like the fact that even Tolkien's Elves seem to break the mold of "Elvishness" (like the Celeborn example you gave); it makes them more real and much more complex. and gives more liberties to fanfic writers