How about things that don't work?
Recording a macro related to charts – broken.
Working with large data sets (large meaning small enough for Excel 2003) – slow, especially if you are charting large data sets.

Did you notice that even after you learn where everything is in the 2007 ribbon, everything still takes longer? More mouse clicks are required, more mouse movement.

Did you migrate existing Excel VBA programs to 2007? 99% of the VBA works the same, but a small fraction of commands work differently. Sometimes it's inconsequential to the final output, like a formerly optional argument is now required. But this still requires testing, fixing, and testing the fixes. Sometimes the changes are much larger, and you need to spend days to work out new procedures.

So far, it seems that the 2010 version will address many of these issues. Not the inefficiency of the interface, but macro recording of charts now works, large data sets do not choke Excel, and some VBA has been corrected. Skip 2007 and wait for 2010.