SEO considerations for teasers and taxonomy

I have been thinking about node teasers and taxonomy settings for SEO, and I'd be interested to hear your thoughts, and about your setup.

In particular I am interested in ensuring there is no duplicate content, and whether taxonomy pages are useful in appearing in search results.

Teasers

Do you display full teasers on the homepage / taxonomy pages, or do you show a trimmed version? I like to show the full version, as it makes scanning much easier, and allows one to see the whole article without opening the node page. But, will having full teasers push the full node page into being considered duplicate content? Which is best?

Taxonomy

Are taxonomy pages good at bringing extra visitors from search engines? Do you block your taxonomy pages from search engines to avoid duplicate content issues?

I have made the basic additions to the robots.txt file, to block comment pages, feeds, etc.
I also use the nodewords module to help with meta data.
I use clean urls for everything.

I'd be interested in hearing any suggestions and feedback you might have.