An Elf with shapeshifting ability? Why?

Originally Posted by Alfirin

My Silmarillion is a bit rusty, But doent's Luthien's shape changing ability derive entirely from the fact that she is wrapped in Thuringwethil's shaping cloak/skin? if this is the case wouldn't anyone who had the cloak be able to shapeshift?

IIRC correctly, Luthien's ability to shift shape does indeed come from her Maian ancestry. This was ability shared by all the Ainur, and examples of it abound. Note Sauron's ability to take the form of a werewolf and Olorin's practice of walking among Elves and Men unseen or in shape like one of them during the First Age. Luthien, as noted by another poster, probably would not have all of Melian's powers, but she would certainly would be expected to have powers beyond those of any other Children of Iluvatar.