I tend to think hardcore relates to dedication and commitment that goes above and beyond what is considered normal (next question is what is considered normal). Being hardcore may be related to but is not the same as being highly skilled.
In general I think hardcore manifests itself in hours played. Either in the form of a lot of hours over the course of an extended period (achievement hounds) or an excessive amout of hours all at once (server first types).
Given how long WOW has been out I think we probably have a large contingent of players who are exceptionally skilled but not necessarily hardcore anymore. This leads me to think that the hardcore versus casual classification should not be used to describe how well one plays their character.