I generally like something that has the following:
-live view (though not as important since most of your work will be in the browser)
-support for JQuery
-Integrated Debugging tools
-Built-in Console and Terminal
Dreamweaver, for instance has "code navigator" which is essentially its version of "firebug"; and even though it is significantly less intuitive (as are a number of development tools in this IDE), the functionality is still there. I also like Aptana because it has many of the features I just mentioned. Additionally, the coding space is just plain pretty.