First of all, Dreamweaver CS4 is enormous. 509MB of sweet, sweet application. Dreamweaver CS3 was a mere 366MB. Coda is just a wisp of a thing at 52MB. TextMate is practically anorexic at 30MB. For what CS4 provides over CS3, I can't really say that the extra 200MB are doing much. Hopefully that's just test code and cruft that will come out before the final version is released.
Looking at the interface, it's not significantly different than the last version of Dreamweaver (true, it's just as customizable as the last one, too). Palettes are organized similarly, with one major difference. The Insert palette now lives down with the rest of the palettes, instead of as a toolbar above the workspace.
One nifty addition is a small tab-like toolbar above the workspace that shows documents (like stylesheets and scripts) linked in the current document. No need to thrash through your document structure to find a buried CSS file: it's listed right next to your open document at the top.
Before I switched to Coda, I used Dreamweaver (in its various iterations, including CS3) since Dreamweaver 3, I think (note to readers: I am old). I'm very familiar with how it works. As I grew in my career, I moved away from the WYSIWYG development process, and started coding by hand. Dreamweaver was a fair-to-middling text editor, and the code view's maturity as part of the Dreamweaver product definitely shows in CS4.
Still, certain features like Coda's terminal integration and visual CSS editor would be a welcome addition to Dreamweaver.
Another addition is the Code Navigator, a contextual menu that shows you the CSS properties of the item you clicked. No more switching to Firebug to find out what styles are assigned to a particular object. A helpful (Firebug-like) addition to this pop-up might be to show which styles are overridden by others.
For me, the most anticipated feature is Live View, a preview environment (much like Coda's) that renders the web page as if it was in Safari, Camino, OmniWeb (thanks commenters!) or the raft of other browsers that use the WebKit engine. The disappointment is this: the Design view (that is, the WYSIWYG environment) doesn't use WebKit, but the same rendering engine as the last version of Dreamweaver. As a result, Live View is given a strange "separate but equal" preview status. I think it would be great to make WebKit the rendering engine for all of Dreamweaver's WYSIWYG output, furthering the push toward design for standards compliance.
Even so, testing is at least easier, obviating the need to switch from IDE to browser and back again.
Overall, Dreamweaver is a mature HTML development environment. The changes to the environment shouldn't have to be radical to be useful. In fact, Adobe is helping the development community push more towards standards compliance, which is great for everyone: developers and visitors alike.