One important thing I feel is missing from the mobile design is the ability to view a tag's excerpt directly on the tag page. I find the excerpts very useful on the main site when I see a tag on a question, but don't exactly know what it's about.
Considering it is an excerpt, it won't take up too much room, which is important for a mobile-optimized design). Like on the full website, it should go below the
Questions Tagged [tag] header, and above the question list. It could even include a
more link that goes to the corresponding
/tags/[tag]/info page. Perhaps something like this: