I don't want to make Andy's life difficult, but here are my observations and suggestions:
1 - it looks like it only looks at the first 3 words; this is too few -- it will never get an accurate list of similar threads just by looking at 3 words, unless you also...
2 - prevent it from searching 'stop words' -- these are common english language words that are frequently filtered out of search engines. here is a list:
http://www.ranks.nl/stopwords/
If you could provide an option to prevent it from searching on stop words, and force it to look up 5 words in the enhanced search, i suspect you'd get a pretty reliable system.