Over the last 3 or 4 years they made their UI progressively worse: replacing spaces for keyword separators by commas, slower autocomplete and now barely visible input boxes. Delicious has been extremely unkind to it's longtime users.
If I'm not mistaken, there were prototypes for doing collaborative filtering of Delicious dataset written by standalone applications. But doing such filtering requires direct access to the database and cannot be done efficiently via HTTP. So, after the current team had bought Delicious from Yahoo, I thought that implementing collaborative filtering would be their main priority. Instead of that, they concentrate forces on worsening and cluttering UI experience along with breaking various Delicious bookmarking plugins for popular browsers.
[1] http://en.wikipedia.org/wiki/Collaborative_filtering [2] http://shop.oreilly.com/product/9780596529321.do