Now it is
New $tag_name questions for $date - Stack Exchange
Could you please replace 'Stack Exchange' with the site name — Stack Overflow, Server Fault etc? It's useful because some tags exist in many sites.
There is a technical problem here, since tag subscription is actually part of a more generic Filtered Questions which allows you to choose more than one site.
For example, this filter takes questions from two sites. What you want to appear in the email subject in such case?
So I'd say just leave it as it is now.