gzip http compression for server pages.
With Icecast 2.4.x there is no gzip compression used for served pages.
While most served pages are not that large, gzip may be the difference between a page being served in one TCP packet vs multiple.
And then there are monstrosity like this http://relay.sonixcast.com/ that would clearly benefit from gzip.
gzip could even be applied to xml stats pages as PHP and other serverside scripting languages also support gzip.
While technical it may be possible to apply gzip to the actual stream I would not advise that, the benefit would be minimal in most cases and the compatibility issues with older players could be high (they may support gzip pages but not gzip audiostreams for example).
gzip should probably be enabled by default so that the "optimal" settings are enabled "out of the box".