[wpm]

Don't ask me, I don't even know what was so bad about XML tags.

[seec]

Not fashionable enough. In all seriousness XML is so much more powerful and no need for a parser with infinite edge case. Editing is just fine with the proper editor/IDE. But there is a religion around plaint text and minimal editors so whatever...

[SpaceNoodled]

Probably the verbosity, necessity of a schema for typing, and mixing of text with tags as siblings.

[nielsbot]

And having to add both opening and closing tags.

[CrimsonCape]

This should be perfectly valid:

< date="today" id="978"/>

I could have used this many times.

[SpaceNoodled]

Yes, that's a large part of the verbosity I was mentioning.