The Open Question argument is theoretically flawed because it relies too much on definitions (see this website's articles on how definitions don't work that way, more specifically http://lesswrong.com/lw/7tz/concepts_dont_work_that_way/).

The truth is that humans have an inherent instinct towards seeing "Good" as an objective thing, that corresponds to no reality. This includes an instinct towards doing what, thanks to both instinct and culture, humans see as "good".

But although I am not a total supporter of Yudowksy's moral support, he is right in that humans want to do good regardless of some "tablet in the sky". Those who define terms try to resolve the problem of ethical questions by bypassing this instinct and referencing instead what humans actually want to do. This is contradictory to human instinct, hence the philosophical force of the Open Question argument but it is the only way to have a coherent moral system.

The alternative, as far as I can tell, would be that ANY coherent formulation of morality whatsoever could be countered with "Is it good?".