Title attributes for major interface elements
Adding title attributes to various important interface elements (the top navigation elements come to mind) would help users new to Redmine to quickly learn something about the way the system works.
Title attributes will be displayed by modern browsers on hover, but could also be enhanced through the use of js libraries like qtip (http://craigsworks.com/projects/qtip/).
This would require that the title attribute text be available in the various translations. It could be useful to allow some administrator level modify this text if needed to match the a particular project's approach to say the Wiki or News.