At the beginning of February I added meta noindex to the tags pages (keyword pages won’t be indexed in Google) to seo bugz.
After Google removed the tags pages from his index (site:seobugz.com), traffic from Google jumped by 300%.
Maybe because the inside PageRank doesn’t split to so many pages and other sites rank better.
Another method to make your tags/search pages not appear in Google is to use robots.txt:
User-agent: *
Disallow: /tag
More about robots.txt and tag indexing read on capslock.ro (in romanian).