Commonly an issue with dynamically driven sites and sites that depend on session IDs, is that you have too many pages indexed. You are thinking, probably, how can I have too many pages indexed? The more pages I have indexed, the better off I am! Well not always. The issue is that when you have a site that isn't well optimized you can run into issues.
Trouble Sites: (1) Session IDs in URL: A session ID would be dynamically generated and appended to the URL of every page. So a unique page about a blue widget might have hundreds of different URLs. For example; http://www.mysite.com/blue-widget.html?SESSIONID=5fd5ds14f56s1fs, http://www.mysite.com/blue-widget.html?SESSIONID=dsf45dsf54sd5s, http://www.mysite.com/blue-widget.html?SESSIONID=5dfs1651ssdffds, http://www.mysite.com/blue-widget.html?SESSIONID=jdfnkfjdsnfkjdfns55 are all the same page, but look unique to Google, because they are different URLs. (2) The same title on every page of your site can sometimes confuse Google, especially when your content is not well indexed or not visible to the search engine spiders. (3) Paginated category landing pages, so you have 10 pages of products within the widgets category, each page may contain the same title tag (i.e. widget). (4) Paginated search engine forums, like WebmasterWorld, page one, page two, ..., page forty-four. All different URLs, but the same exact title tag, yet different content on each page.
But often this is not a major issue with the search engine. They all have built in ways to handle what they might call "duplicate content" and filter them out. I would be more careful with issues one and two listed above then the others.
Forum discussion at Search Engine Watch Forums.
