Confession time...
Something that will often irk me is sound effects, or rather... the lack of them.
A comic doesn't have to have a "WHOOOSH!" or a "KABAM!" to be a comic but if it does have them, I tend to enjoy the book more. Especially if they look organic and compliment the art.
BTW... I remember when I was younger, it was a sort of unspoken rule that if a character had a thought balloon they were more important than the ones who didn't. Kinda like a reverse Redshirt on Star Trek.