I have just been reading up on the topic and found this guide:
Possible "Speed Bumps" for SE Spiders:
* URLs with 2+ dynamic parameters; i.e.
http://www.url.com/page.php?id=4&CK=34rr&User=%Tom% (spiders may be reluctant to crawl complex URLs like this because they often result in errors with non-human visitors)
* Pages with more than 100 unique links to other pages on the site (spiders may not follow each one)
* Pages buried more than 3 clicks/links from the home page of a website (unless there are many other external links pointing to the site, spiders will often ignore deep pages)
* Pages requiring a "Session ID" or Cookie to enable navigation (spiders may not be able to retain these elements as a browser user can)
* Pages that are split into "frames" can hinder crawling and cause confusion about which pages to rank in the results.
Possible "Walls" for SE Spiders:
* Pages accessible only via a select form and submit button
* Pages requiring a drop down menu (HTML attribute) to access them
* Documents accessible only via a search box
* Documents blocked purposefully (via a robots meta tag or robots.txt file - see more on these here)
* Pages requiring a login
* Pages that re-direct before showing content (search engines call this cloaking or bait-and-switch and may actually ban sites that use this tactic)
Link to guideDud