Improve performance, make it work on Joomla 1.5, and later the newer Joomla version(s).
Simplify installer UI by only showing information that is necessary, hiding what isn't (stuff like PHP version check isn't necessary unless the server fail to meet the minimum requirement).
Create Phing script that works out of the box with a full nooku-components svn checkout on the trunk or a branch, but also have the phing script work independently by doing an svn export of com_koowa off the assembla repo whenever a local checkout isn't available.
Make sure that installer rollbacks (on Suhosin failures, server failing to meet minimum requirements and such) properly and never leave a broken install behind.
Show a report of what happened as a result of the install, like showing created folders and files, changes settings etc (nice to have).