these are some excerpts from Google's own "Quality guidelines - basic principles":
- Make pages for users, not for search engines. [...]
- Avoid tricks intended to improve search engine rankings. [...] Another useful test is to ask, "Does this help my users? Would I do this if search engines didn't exist?"
- Don't participate in link schemes designed to increase your site's ranking or PageRank. [...]
as the issue you've concerned is clearly a direct consequense caused by violating those basic principles of Quality Guidelines,
and since it's very unlikely that Automattic Inc. will ever
Use robots.txt to prevent crawling of search results pages or other auto-generated pages that don’t add much value for users coming from search engines.
to adjust their global tags pages along with Google guidelines policy,
I'm afraid that if you want to continue stick your blog with wordpress.com, then you'll have take this issue with Google itself.
otherwise I cannot see any other meaningful way to resolve your problem, except deprivng yourself from tagegorizing your posts or moving to another free bloghosts, which are not that big into SEO gaming.
and since so everyone reading this post is on the same page, please let me add the following just to finally dispel any possible doubts or apprehensions related with alleged "google-juice" from GT, induced by so many BS tales around here.
so I think you should know that all of the so called "Top WordPress.com blogs today" you can see on your dashboard everyday (which are in effect so called "VIP blogs") have they tag links all go local. they don't participate in the wp.com global tag system, despite they are obliged to even.
I bet since those .com-hosted blogs are running ads of their own (not the ones automattic is plastering on yours), they don't have any reason to give up a google-juice they earned into global tags system.
that's so just because this particular google-juice of theirs is the *real* one, not that an entire fabrication squeezed out of your blogs or may be even from a mythical camel cheese ;-)