At this moment, almost 10.000 plugins are being offered via WordPress.org The search filters that exist are fairly adequate. However, it would help many users if one could filter on compatability.
For this, the user has to be able to select a version of WordPress from a combo box in the search filter area. Only plugins which aren't reported to be broken should be returned (including not enough data). If no selection is made, this filter will not be activated.
Implementing this will improve search results and motivate plugin maintainers that their plugins are compatible with at least the latest stable version of WordPress. Because otherwise, changes of getting noticed by a users will be more slim.