@Tea: Mono-Light can kill Mono-Dark pretty easily due to Holy Light. But it's uselss against anything that's not Darkness or Death. You'd arrive at this conclusion if you really think about it.
To me, being a Master of an element means you have to know that element. You don't have to have godly decks, but you must have a completely thorough understanding of that element, its cards, its strategies, its synergies, and everything related to that element. To me, it's the knowledge that matters.