You are right about most of the missing features. I’ll be addressing most of them in the September release.
However, the following 2 features are already there:
- Fielded search (missing in all social bookmark managers)
- Full-text search (or I can’t find it)

For fielded search, just search with “field:text-here”. For example, title:rss
Here are the results:
http://www.simpy.com/simpy/User.do?username=otis&q=title%3Arss
(Note the number of hits - 7 - and all links have RSS in the title. Some may also have rss in the full-text content or in tags, but the match comes from the title field.)
There are several fields you can use (listed in the FAQ), and you can use any combination of them to form really precise queries.

The full-text search is also there. If you look at the results for the above URL, you will see dynamically generated snippets in a KWIC format (KWIC - Key Word In Context - an Information Retrieval term for snippets around original query terms), with search terms highlighted. These snippets come from the full-text content.
Another example:
http://www.simpy.com/simpy/User.do?username=otis&q=rss

Note the number of matches there - 151 currently. This tells you that this searches more than just link meta-data.