Hobgoblins' eye changes based on whether or not it can use Thermal Barrier. I've killed well over 10,000 Hobgoblins and yet not once noticed this small detail until I was just mucking about with the HUD off.
[spoiler]Left: On cooldown
Right: Active[/spoiler]
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*Bungie makes new content* This sucks. *looks at grass* WOAH MAN THE AMOUNT OF DETAIL 10/10 THEY PUT TOO MUCH EFFORT INTO IT BUNGIE YOU DA BEST