XML, what to avoid

Hello list,
I'd like to know if there are XML technologies/practices that are best
avoided because there's a general consensus or empirical knowledge
that they are:

- obsolete, outdated, replaced by something newer/better
- unimplemented, unsupported
- bad for performance
- bad for long-term maintainability
- bad for usability
- bad for extensibility
- other

--
Max Toro