I'd like a documentation tool that uses a less verbose markup. Filling my comments with XML just seems like an unnecessary eye sore, and a huge waste of key strokes should I have to edit something outside of VS. I spend a lot more time in the ruby world and would love to find something more like YARD. I have a feeling this is a pipe dream, but I figured I'd give it a shot...
I personally really like Doxygen. That being said, outside of its own format, it also supports XML format (and others). I've had great luck in getting Doxygen to play nicely with large C# projects.
XML documentation is what is used by Visual Studio's IntelliSense. If you want the documentation of your code to be visible to other developers using your classes and methods, it's the way to go.
Tools like GhostDoc can automate the process of creating xml-doc comments; in most cases its documentation is a little simplistic, but it gives you all the elements needed to describe the class, property or method that you can customize as necessary.
If you simply cannot tolerate xmldoc and don't care about IntelliSense tooltips, you can create multiline C-style comments, and VS will automatically format them as an asterisk-bound block. This is useful for file headers, but you can do it anywhere in the file. I'm sure there are tools that will allow you to specify context-sensitive templates for C-style block comments.