Severe health issues are found on your site
There's nothing in your robots.txt file that is out of the ordinary and for the most part you probably want to keep all of those exclusions, checks, etc. to save on bandwidth and for security. The health message you're seeing is likely due to another issue within the site or maybe just a temporary Google hiccup that will resolve itself shortly. Are there any other messages within the Google Webmasters page that indicate other issues, ex. permission problems?

Another possibility is to submit an XML Sitemap to Google directly to tell it exactly what you are expecting it to index:

http://drupal.org/project/xmlsitemap

I've found that this resolves some Google crawl issues in the past.