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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You didn't kn- Oh wait. Something actually new. Carry on. [spoiler]I thought it was a damaged indicator. I noticed it disappeared after the barrier was used, and they never survived long enough to use it twice, lol[/spoiler]