The "feature" of cloning an issue with sub-tasks now adds the prefix to all cloned subtasks. This breaks usage patterns where a person doesn't want those cloned subtasks to have a prefix, and it goes against the expected documented behavior.
http://confluence.atlassian.com/display/JIRA/Cloning+an+Issue
http://forums.atlassian.com/thread.jspa?threadID=43913&tstart=0&messageID=257341990

From a severity perspective, it eliminates our ability to use jira for test case management, as every new release produces a few hundred clones of subtasks.