ZydoSEO - 1:07 pm on Jun 8, 2013 (gmt 0)
Blocking 750 of 800 indexed pages could drastically affect your site's traffic and ranking. That's 94% of your pages.
If the web pages in those directories which you've blocked have a significant number of inbound links, the link juice coming into the blocked pages can no longer be passed around to the various other pages on your site. Blocked pages cannot accumulate PageRank/link juice, and since they can't crawl the pages to find outbound links to other pages on your site, those internal pages linked to from the blocked pages are no longer getting passed PageRank/link juice.
Also, if a lot of those blocked pages were actually ranking for various keyword phrases, by blocking them from being crawled you will have killed your rankings for those phrases (some less relevant, lower ranking page my now be ranking but much lower) and thus the traffic will also have diminished.
Just a guess...