To what I understand till now about Web 2.0, it emphasize on Search Engine Optimization as one of the main area of concern. Let me try to list down few of the Search Engine Optimizations from CMS implementation point of view:
A utility to detect duplicate content detection at the time of content publish
I do not think that there is any WCM out in the market (commercial or Open source) that provides any utility for duplicate content detection
A spell-checker to ensure that the content submitted online doesn’t have any spelling mistakes
This has become and integral part of WISIWYG editors like FCKeditor. Most of the WCMS tools adopt to such content editor tool.
Content Structure which enforce Alt attribute for Image tag, title, meta descriptions, keyword tags and templates supporting picking up those information while rendering page
A more of a design consideration and rendering aspect. Lets leave this to individual design and implementation. It might be a good idea if in future, CMS provides something out-of-the-box for such considerations
What You See Is What You Get (WYSIWYG) editors for providing facility to content editors to add formatting to text content
Most of the WCM provides WYSIWYG editors that provides basic formatting stuff like h1, h2, b etc. OpenCMS has its own inbuilt editor.
An editor that can clean up unwanted HTML tags and make it W3C compliant
Its again part of the WYSIWYG editors.
An editor that gets integrated with your CSS of the site
Most of the text editors provide this facility. FCKEditor is one such that support CSS for better integration with website. WCM’s are adopting these editors for their content editors frontend.
A utility to help detect duplicate page titles or a mechanism to generate unique page title
I can not re-collect that there is any CMS in the market that provides such utility. At present its taken more at individual implementation level rather than a CMS providing out-of-the-box.
A utility to determine broken links at the time of content publish to avoid broken links
OpenCMS is one that does provide this functionality of validating the links and reporting the broken links. but I do not know many other which does that. Fatwire do provide this in a little different way, if at the time of rendering content, it doesn’t find the link, it removes hyperlink and render content. But I think most of the CMS products doesn’t really provide this out-of-the-box.
Looking at my list above, I feel most of the Search Engine Optimization techniques are taken care at the text editor level which have become powerful over the period of time. From CMS perspective, providing few utilities like duplicate content detection, broken links etc. will sure make them Web 2.0 compliant. 
