Well, I can't say I like Dreamweaver better than Nvu, as I've never used Nvu (but I intend to check it out now), but here are a few things I really like about Dreamweaver...
CSS Layout Backgrounds: Visually (using colours) shows you the size/position of CSS elements, allowing you to resize/reposition them just by dragging them.
Tracing Image: Allows you to place an image 'in front off' the design view of your page (with configurable transparency). I find this is great for converting photoshop mockups to real pages.
Code Collapse: Being able to hide/expand sections of code while working in code view.
There is a lot more I like about Dreamweaver, but those are the things that come to mind.
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