[Gwaihir the Windlord]

Please don't heckle my ignorance too harshly.

Well, as yet you've shown none. Quite a good point there, actually. Welcome to the downs! You've come just in time to see the new board.
(I like saying that [img]smilies/smile.gif[/img])

Here is the quote, for convenience sake.

And his face was terrible to behold.

It may mean in a metaphorical sense. Eru is a spirit, hanging the air sort of thing, you say; if so, his mood would have been able to be felt by those around him. If you asked them to describe how they felt it, thye wouldn't be able to say.

Then again, Illuvitar might have created the Valar to be more or less the same shape as us. Able to change, of course; but the human form is obviously their most favourite form, probably the natural one that Eru created them as. Then he appeared to them himself in that form, so they could see him clearly.
Perhaps he appeared to the Valar in a human form because that is what they would most understand.