The new release overhauls the suite's icons, which haven't significantly changed since Office v.X appeared in late 2001 with bubbly translucent icons intended to reflect the Aqua appearance of Mac OS X.
The new 2011 icons are toned down and more serious looking (below). Also refined are the splash screens for each app.
The new suite works to bring more visual harmony between the Mac and Windows versions; Microsoft just released Office 2010, the Windows equivalent of the upcoming 2011 Mac version.
Additional coverage of the latest Office 2011 and the suite's historical development:
Road to Microsoft Office 2011 for Mac: A New Hope
Road to Office 2011: New looks, support for Exchange, VBA
Road to Mac Office 2008: an introduction