With Melkor being my favourite Vala (I know, who would've guessed it?
) I am frequently dismayed by the way mortal artists depict his looks. In fact, the majority of art in which Melkor's face can be seen have him look like some disgusting ghoul or mutant thing (I remember one artwork in particular in which Melkor was portrayed as something that would fit in quite nicely in an undead army, having no nose and all that (that would be David Days' Bestiary, for those interested)). But when one takes into account Melkor's personailty one must ask oneself this: Why would Melkor, who by all accounts could be considered vain, make himself a form that was ghastly to behold?
" Ah," you say, " But the text clearly says his form as the Dark Lord was 'terrible'! Clearly this means his form must have looked gruesome!" But lets take a closer look at the meaning of the word terrible. Though today it's usually used as a negative desciption, the word terrible used to have another meaning, more akin to 'awe-inspiring'. Take for example Galadriel's quote '....Beautiful and terrible as the dawn....' Clearly the dawn is not gruesome, ugly or disgusting to behold, but it's certainly awe-inspiring!
I believe that this is exactly how Tolkien meant his desciption of Melkor's guise as the Dark Lord, a shape of unrivalled dark glory and power, not some disgusting thing that looked partially decomposed. Granted, the scars of battle would make him unpleasant to behold (if the sneer of contempt for every creature apart from himself and his gaze which could break all but the most strongwilled person in an instant didn't do that already), I see nothing in the text to support the silly notion many artists seem to have that Melkor's facial characteristics would resemble that of a gorilla (Melkor vs Fingolfin, Ted Nasmith)