for many many people, as for me as well, a changelog is a very important thing; it is all about justifying to your users why they should upgrade to the latest version of a plugin.
The plugin Changelogger shows the latest changelog right on the plugin listing page, whenever there's a plugin ready to be updated.
To the plugin: http://wordpress.org/extend/plugins/changelogger/
Here's a screenshot of what the plugin's doing: http://wordpress.org/extend/plugins/changelogger/screenshots/
Hope it's a helpful improvement for those using and updating plugins. :-)