A recent discussion of wellbeing over at Go Grue reminded me of how easily philosophical debates can degenerate into mere terminological disputes and "concealed tautologies" (as Sidgwick and Parfit warn against) if we're not careful. For example, a hedonist might be tempted to define 'wellbeing' as 'the felt quality of a life'. But then they are not actually saying anything of substance when they assert that 'wellbeing is the felt quality of a life'. Given what they mean by the former term, this claim becomes logically equivalent to the empty tautology, 'The felt quality of life is the felt quality of life,' which is hardly a claim worth making.
In order to make our affirmation of a particular theory of wellbeing substantive, we must have some independent grasp of the meaning of the term. For example, we might analyze S's interests in terms of what's desirable for S's sake. But even if we lack a full-blown analysis of the concept, we should at least have some sense of its conceptual role, i.e. what implications it has for other philosophically important concepts.
I think it's very dangerous to just test one's linguistic intuitions in isolation. Often, people seem be drawn to hedonic (or 'mental state') theories of wellbeing simply because they think it sounds wrong to say that someone was "harmed" by an event that they never learned of (or that otherwise impacted their subjective experiences). This does not strike me as a philosophically interesting or relevant consideration. Instead of assessing what combination of words sounds natural to our ears (as if we were mere grammarians), we should be assessing the real content of the claim, as revealed by its philosophical (e.g. normative) implications.
The concept of 'wellbeing' seems important only insofar as it has implications regarding what we have normative reason to want and to do when we care about a person (including ourselves). If we do not think that a loving parent would (qua loving parent; bracketing all other motivations) want to plug their child into a Nozickian experience machine to live a meaningless life of pleasant delusion, or that a prudent person would want this for themselves, then we shouldn't be hedonists about wellbeing. It's that simple.