Particularly annoying is that there's no way to do lists with attributes. Looks good:

  <user name="alice" group="wheel" />

Oh no:

  <user name="alice" groups="wheel admin sudoers" />

Or is it one of:

  <user name="alice" groups="wheel,admin,sudoers" />
  <user name="alice" groups="wheel:admin:sudoers" />
  <user name="alice" groups="wheel;admin;sudoers" />

Or give up and use elements:

  <user name="alice">
    <group>wheel</group>
    <group>admin</group>
    <group>sudoers</group>
  </user>

Hold on, should there be a container?

  <user name="alice">
    <groups>
      <group>wheel</group>
      <group>admin</group>
      <group>sudoers</group>
    </groups>
  </user>

XML really needs either richer attributes, or no attributes.