I like the search engines and discoverability. My own project - much, much smaller - is for embedding search into the Javadocs themselves (along with some SEO improvements, like iframes and better meta-info).

The search is driven by Apache Solr and can be seen at: http://www.solr-start.com/javadoc/solr-lucene/index.html . I use custom doclet to generate the actual Javadoc, which is an interesting challenge all by itself.