wounded in the line of duty
While it's true Drupal core will not provide all the features, there are many contrib modules to fulfill the requirements.
A: done by default by core
B: don't know how it compares to other systems in terms of scalability, but I don't see why it would not. see drupal.org forums.
C: LDAP contributed module: http://drupal.org/project/ldap_integration
E: UTF8 is default in Drupal core. i18n support etc...
F: security: I think it's quite good. disclaimer: I'm in the sec team :)
G: lots of themes http://drupal.org/project/Themes
H: I don't know of anything specific to forums, but there is the import/export API.
Other forum specific applications might have more features than Drupal out of the box, but if you plan to extend your forum into a multi feature site (like a portal), Drupal is definitely worth considering.
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