With XML and other "fully qualified" syntaxes the named closing tag makes it easy to visually see where a block starts and stops. Humans parse the text too. This begs the question: Do you read config more or do you write config more?
Braces and parens all look the same, so its harder to visually match them. "But wait," you may say, "my editor / IDE does brace and paren matching so I can easily see and jump to the corresponding brace or paren." True, but doesn't your editor / IDE also support or have plugins for automatic HTML / XML end tag insertion?
Personally, I am equally comfortable with XML and JSON style configuration, but I find the brace-and-quoteless style unsettling because I believe that whitespace is a poor choice for delimiting structure. Your opinion may vary and that is fine. Do what works best for you.
In my opinion one of the weaknesses of XML is that it gives so many ways to express the same data, in attributes, as nested elements, with CDATA, and probably different other ways. And often it is a matter of taste. So in the end you come up with so many approaches to express similar structures.
>> So in the end you come up with so many approaches to express similar structures.
Expressiveness and flexibility are good so that you can define configuration to meet the needs of the system or application you are building.
XML Schema, Relax NG, etc. can be used to specify how the XML configuration should be structured, limits on data types, required versus optional configuration items.
As I said before, XML gets a lot of flack for being verbose, ugly, and complicated, but it is mature, widely-supported, and might be worth considering depending on your needs.
Moving up the enlightenment chain:
or its brace-and-quoteless friend: place a lot more emphasis on the payload and less on the packaging