XML is far too heavy, agreed, and it's signal-to-noise ratio is abysmal.
I think that using a Lisp (or Lispy-looking) format would be extensible,
easy to parse, and make the most people happy.
Allow me to toss out JSON. It is about as light weight as using S-EXP,
...else along those lines anywhere else.
The problem I have with JSON (and to some extent, Lisp) is that it is
not self-documenting. You can't open a JSON document and immediately
see what everything is and what it does; it just looks like gibberish
and brackets.
Also, it doesn't require a consistent newline scheme.