I'd like to relieve you of any anxiety you feel about this.
The global tags pages now referred to as Topics have been broken for a long time perhaps due those legions who spam it and who try to game it everyday day in and day out. Staff decided to focus on integrating the topic browsing experience with the Reader and to discontinue the old listings.
The flow of traffic from the Topics pages to blogs is negligible. The flow of traffic from Google and other search engine is significant.
Search spiders are focused on keywords found in content. Tags are not required for SEO purposes and Matt Cutts of Google makes that clear Revisiting Keywords and Tags
These 5 videos introduce how Google discovers, crawls, indexes your site’s pages, and how Google displays them in search results. http://onecoolsitebloggingtips.com/2010/03/22/5google-webmasters-video-tutorials/
Need a short introduction to the basic elements that make your website attractive to search engines? Don’t know what search engine optimization is? Know someone who needs easy to understand overview of SEO? That’s what these videos provide, in three minutes. http://onecoolsitebloggingtips.com/2011/10/21/two-seo-videos-for-bloggers/