The most important for me is better syntax. This is what haml should have been and I fully expect it to replace haml after it gets ported into other languages. After writing with both, it was an easy choice honestly.
It's also got a couple extra features, but haml could match it.
I'm curious, too. The syntax looks very similar to HAML but isn't HAML. Wouldn't it make more sense to adopt HAML itself instead of inventing a dialect?
