HAML is fantastic overall, but there are two ways in which I don't think it's addressing the same problem as something like this:

* Whitespace-sensitive means I don't want to use it in a text-field in a browser -- in an editor I can have guide lines to show me the indentation levels and what lines up with what, and keyboard shortcuts for indenting/unindenting blocks quickly.

* Nesting and inline stuff just doesn't work well since you'd end up with stuff like a %b on one line and then an %i on the next line unless you just went plain-HTML for the line. HAML is much more suited for page structure, and not so great for inline content.

Something like the following line is really cool with a quasi-TeX feel: "Using multiple tags: {b {i This is italics and bold.}}"