When searching for plugins on wordpress.org, very often the first couple of results are old and not maintained any more (e.g. try the search term "rating"). Some few plugins might work unchanged since 2005, but most of them stop rather soon after maintenance is dropped. To make finding useable plugins easier, the search really should be improved.
 Archive older plugins and exclude them from the normal search (you could check a checkbox to search for archived plugins, too, but leave it unchecked as default). Define "old" as "marked as compatible for a version three or more versions behind the current version". So if WP 3.0 is out, plugins which are marked as compatible up to and including WP 2.7 would be retired to an archive.
 Alternatively at least make the compatibility to the current WP version a very strong part of the relevancy criteria (=default search sorting), so that incompatible plugins don't get to the top search results.