Verbose, unwieldy, harder to read by humans (especially without syntax highlighting) in terms of picking out key data. More unwieldy to write in most languages as well (most now have libraries that will convert a dictionary structure to [or from] JSON in a single call, while XML often still needs to be assembled element by element).

I think it would be an exaggeration to say that there's anything "wrong" with XML, but there's a reason JSON has by far overtaken it outside of "enterprise" applications.

That said, in some ways XML is preferable for standards - particularly in that there's a standard way to publish a document type specification, and widely available tools for validating a document against such specifications. The somewhat experimental equivalents for JSON haven't really ever caught on.