In regards to the death of Baldur, one current theory is that the name "mistilteinn" is the name of a sword or other weapon. By the time the christian monk Snorri translates the myth he has confused several earlier tales into one.
In an article be Professor Eric J. Sharpe on Scandinavian Mythology it says about the killing of Baldur: "Frigg said that there was only one thing, a plant called mistilteinn, which seemed too young to take the oath. Loki found the mistileinn and pulled it from the ground" This myth appears to be a variant of the theme of the dying god of vegetation, though without the element of his annual return to life. The
mistilteinn though, can hardly have been the mistletoe, as had long been supposed.. Rather it seems to have been a weapon."
The legends say
mistilteinn which was "pulled from the ground", not mistletoe, which is a parasitic plant which grows on trees.
Also this article:
http://www.northvegr.org/lore/rydberg/101.php talks quite a bit about an earlier saga where Baldur is a demi-god and Hoder is the hero, who must quest for the magical sword mistiltienn, which is the only thing that can kill Baldur. Yet even this translation contains contradictions.
Basically, even scholars aren?t certain what it is.