There is so much stuff in SharePoint Designer 2007, it's hard to pick favorites, but if I had to choose the five things I'm most excited about...

1) Data View v2:
In addition to update (form) support, we can also now create "aggregate" views across multiple sources, have a cool new Xpath "Expression builder", integration with the "Business Data Library", not to mention a nice UI for managing view parameters. 

2) Workflow Designer:
Leveraging the new "workflow" capabilities built into the SharePoint platform, we added a "rules-based" designer to SPD that allows you to quickly add application logic to your SharePoint apps. Think of it as a way to put code behind your apps. I'll be talking about this feature lots more soon :).

3) CSS support:
We built a whole slew of features to create and manage CSS styles. We also worked with the SharePoint team to make it possible to 'detach' from ows.css (now called core.css) and easily customize the SharePoint stylesheet.

4) Better code view:
CSS/XSLT/Xpath intellisense, code view/design view sync for Data Views (best feature ever!), CSS hyperlinks, and more.

5) Rich ASP.net integration:
We collaborated with the Visual Studio team to build really deep support for ASP.net into SharePoint Designer, e.g. great previews in design view, full property grid support, as well as support for bringing up the native "builder" dialogs.

Welcome to the list of features I plan to blog about over the coming months :).

oh ya, did I mention we also built "Re-Ghost Page"?!