Balrogs

[Dondagnirion]

"From this description it appears that a Balrog's form could be that of a really tall child of Eru." - Nils

Not quite. A child of Eru is an elf as we all know and Balrogs in no way look like elves. A Balrog is as Gandalf said "a demon of the old world." (I believe the quoute is something like that). They had lost their power to change form because when they assumed the shape of a Balrog they gave up something of themselves that was present before they allied themselves with Morgoth. It is similair to how Sauron lost his ability to assume any shape after he was caught in the storm Eru made to destroy Ar-Pharazons armada. There is never a reason stated why the Balrogs lost their ability to change form so much earlier than Sauron but I like to theorize that it was because he was a stronger maiar than they. It may also have to do with the fact that Sauron was not wholly evil until he rejected Manwes summons (Manwe was going to pardon him) in fear that Manwe was going to throw him in that endless abyss deal with Morgoth.