Tim Bray re-phrased the now-classic Metcalfe's Law into something that XML spec writers can appreciate:
"The value of a markup language is proportional approximately to the square of the number of different software implementations that can process it."
Yeah, yeah I know its an old post but I'm way behind on my blog reading (and posting for that matter!).
While I wholeheartedly agree with this assertion (perhaps someday to be known as Bray's Law) I don't necessarily agree with the markup languages he identified as "the big five".
Read the whole post here.