* Readability can be somewhat subjective. Neither Coffekup or any of the markup DSLs I've seen are inarguably much more readable than HTML. A few extra braces to closing tags do not immediately make something less readable to me, and the example given does not seem like a clean win to me. Better in some cases, worse in others, a wash overall.
* I do not think "portability" means what you think it does. Coffeekup appears to only run under Coffeescript. That makes it rather un-portable. Many other template languages have multiple implementations in many languages. That seems unlikely with Coffeekup.
* Reducing formatting errors would presumably be because of static syntax errors caught by a Coffeescript-aware editor, and dynamic errors caught by the Coffeescript compiler and JS runtime. The same is achieved with a HTML-aware editor and various available validators.
* I don't see how Coffeekup that generates HTML is easier to learn than just HTML.
* Or screw up the hierarchy because of whitespace errors. This is just the significant-whitespace debate brought to markup. If you want significant whitespace for your markup, I'm sure there are portable template languages with significant whitespace.