easy integration in existing systems
tag whole projects (this is what the other tagging-systems do)
tag single files
tag collection of files
tag source-parts within a source-file
provide freely definable tagging-classes (like functions, packages, directories, .. )
distinguish between tags from everybody to those from project-owners (maybe also from registered/unregistered users)
web-interface for tagging
code-comment system to add tags (developers only of course) maybe like javadoc?
relevance of tags (defined by developers? defined by number of submissions?)
date/time of tag-submission
removal of tags
automatic take-over from tags to new releases/files?
upper/lower case auto-detection for tags (e.g. linux was submitted 5 times and Linux 300 times, then the tag would appear as Linux)
filter-plugin-interface (e.g. to add a stop-word filter for spam-words)
templates for tag-displaying (e.g. add <href or class-attributes for each displayed tag)