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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Now that you say it, I guess I kinda just knew which one was which but never fully processed the glowing. It's always been a thought for me, like "pew pew pew and wait. it has a shield" or "pew pew pew, it'll die. "