IT'S THE EYEBROWS

I've always wondered why Discord always looked more intimidating in his premiere than anywhere else to me, but I couldn't figure it out for the life of me even when comparing images from episodes.

BUT I'VE REALIZED!
His eyebrows are consistently higher on his head and usually not even attached in the premiere, as well as somewhat larger, while in all later appearances they've started right above his eyes, only going higher if he's surprised or excited. They were also much less mismatched than they are now.