What can't Joomla do?

Not much! It would be nice to say Joomla can do "anything", but it can't - at least not yet...

If Joomla has a weakness it is its lack of native "granular" ACL user management in the current J1.5.# releases, although there are a number of add-on components that can provide this. Joomla 1.6, currently under development and due for Beta release late in 2010, will have full ACL capability.

In practice this only affects a very small number of sites, as most sites only require the provision of a general private members area.