WebMatrix tends to reformat your code (as does Visual Studio), does not allow you to specify tabs for indenting or allow you to customise the syntax highlighting. Also WebMatrix does not like Dreamweaver code and has not been updated in a while.
I like to keep control of my code formatting and style and not have an editor reformat it (unless I specify it to)
It also can not compile code (which is why I use SharpDevelop).
So there is no Visual Studio.NET Web Developer Edition then? Pity, I am sure people would use it and it should cost less that the Pro edition (due to lack of Winforms support)