Something that bugs the heck out of me is when fantasy writers make up their own heraldry and it sucks. I confess, I was a herald in the SCA years ago, and it made me obsessive about the quality of heraldry in fiction.
I don’t demand that the rules of heraldry be followed absolutely (there are multiple systems, for one thing), nor that fictional heraldry maintain enough difference so as not to conflict with existing heraldry. That would be absurd.
All I want, and it isn’t much, is for the founding purpose of heraldry, specifically in a battlefield context, to be remembered. Heraldry is a means of identification. When warriors are armored head to toe, the pictures on their shields are the only way to tell friend from foe.
It makes me nuts to read descriptions of beautiful banners that couldn’t be made out from a distance of more than five yards. Take a look at these two shields. Which one would you be able to read from half a mile away, in motion, through a forest of spears?
I rest my case.