What made William the Conqueror's coat of arms distinctive?

It's absence! William the Conqueror died in 1087, fifty or so years before heraldry was invented in 12th century Europe.

Miles, the answer that I found was that William's coat of arms had the bar sinister on it, a diagonal bar going across the coat of arms that meant William was an illegitimate child.

See my source here. Information is found in the third paragraph from the bottom.