Why are bookmarks important? You always find interesting sites, some of them are helpful, other just funny. A year later you remember there was a site that had ironic pictures about work, pessimistic slogans and even something you could buy (can you find the site?).
Google's search within your bookmarks isn't enough to solve the problem, because Google searches the content of the sites. And most sites don't contain an in-depth description of themselves.
Del.icio.us searches the tags associated with each site, the title and the description, but it doesn't search the content of the site.
The solution would be to see the tags and the description provided by users as metadata, the number of bookmarks as a measure of popularity and the users who bookmark a site as a recommendation profile. If you visit a site and don't bookmark it, or hit back within a couple of seconds, the search engine would give a negative vote.
PROs:
* find what people with similar interests think it's important
* discover more small sites
* rediscover sites you've visited, but don't remember too much about them
* the index would change often
CONs:
* many people should be involved, so that the data is not skewed
* popular sites will be even more popular
