Now you can edit the description meta tag without editing the template. If you edit the description meta tag for the entire blog, you can also write descriptions for your blog posts. This is useful because Google's snippets sometimes rely on this meta tag.
For the first time you can create a custom 404 error page for a Blogger blog without buying a domain. Just "enter an HTML message that will be displayed on the Page Not Found page instead of the generic message." Google has some tips for creating useful 404 pages and there's even a widget powered by Google search that shows related links and a search box with appropriate search suggestions.
You can also create custom redirects, but only for internal URLs. For example, you can redirect http://googlesystem.blogspot.com/mailto to http://googlesystem.blogspot.com/2012/02/open-mailto-links-using-gmail-in-google.html. Make sure that you leave out the first part of the URL (http://googlesystem.blogspot.com) when you create the redirect.
There are also options for customizing the robots.txt page and robots header tags. It's probably a good idea to use Blogger's robots.txt page as a template (http://yourblog.blogspot.com/robots.txt) and only add some new pages you want to be ignored by search engines.