Interesting study quoted in SearchEngineWatch about a site that consisted entirely of dynamic URLs which were remapped to static URLs - nothing ….and their Search Traffic doubled while overall traffic nearly doubled (see below).

"…a massive URL move involving every page on the web site except the home page was completed without incident. Everything was resolved by the search engines in a matter of weeks. This is a clear example of how this should be done."

Site Traffic

Date	Visitors Per Day
11/1/2006	9,000
1/3/2007	12,000
3/1/2007	13,500

Search Engine Journal, which quotes the story, says that only the URLs were changed, nothing else - but that's not true, as the SearchEngineWatch study does mention SEO changes that happened at the same time the URL re-write was going on (strictly speaking - it would have been better if the SEO changes were done a month or two after the URL re-writes - just so we could see for sure what part of the traffic increase might be due only to URL changes).

I see improvements like URL re-writes as part of the 80/20 rule; 80% of your improvements will come from 20% of the changes made (or something like that - it can be the 90/10 rule too).   Other changes, like optimizing body text and anchor text on several pages might have less of an effect, overall, on search traffic than just fixing the URLs.

Often, I've found the technology a site is built in, often the result of uninformed choices made before SEO was considered a factor, have more to do with why sites don't rank well that should be doing better, than anything else.  And I'd add, with a large site, like IBM, for example, just a simple change that helps URLs get crawled better can have enormous effects.

http://www.google.com/search?hl=en&q=allinurl%3Aibm.com

I remember a couple of years back when that query only showed a couple of million URLs when it now shows 22 million.  Not that the change was easy to make - it's not - but make changes that led to more pages being crawled had nothing much to do with optimizing the pages for SEO, it was an infrastructure change - sometimes a simple code change on robots.txt on a couple of servers or something similar to that.