The autocomplete feature in autotext. You used to be able to create custom autotext entries that would autocomplete while typing. Now you have to insert them through the regular autotext process. When creating a form, if you use a “legacy” form field, you have to save the file with a legacy file extension in order for those to work, but then the new form fields don't work, so you have a choice of which fields you'd like, but you can't use them all. And of course, the help feature doesn't bother to tell you that, I found it in some obscure book I was researching. And don't even get me started on what they did to master slides and themes in PowerPoint.
For the record, I do professional development training in both these versions of MS Office. Did in 2000 and 2002, too. 2007 is not designed for power users. It's designed to appeal to the lowest common denominator.