The fundamental objection to such a view is that it commits the so-called naturalistic fallacy.
It might be easier to understand what this fallacy is by means of an analogy. Suppose someone claimed that “true” means “stated in the Bible.” Now, that can’t be right – and one way to see that it can’t be right is that it makes perfectly good sense to say things like “I know that x is stated in the Bible, but is it true?” If “true” were synonymous with “stated in the Bible,” however, then such a thing would not make sense: it would be like saying “I know that x is stated in the Bible, but is it stated in the Bible?” The word “true” doesn’t mean the same thing as “stated in the Bible”; that’s just not how it is used.
Similarly, if “good” meant the same thing as “well-being,” then (as Harris implies) it would never make sense to ask whether well-being is good: it would be like asking whether well-being is well-being. And yet the question “is well-being always good?” does make sense.
To convince you of this, consider the following thought experiment, which was first proposed by the philosopher Robert Nozick and which is discussed in the appendix of Harris’s book: Suppose that there are alien creatures who would experience an increase in their well-being by devouring us that surpasses any amount we would lose. That is, their gain in well-being would more than make up for our loss. It follows that if we became their dinner, the amount of well-being in the world would increase. Now this means that, if increasing well-being is always a good thing (which would follow if "well-being" is the same as "good"), it would be a good thing for us to be devoured. And that doesn’t seem right – not to Nozick, not to me, and probably not to you. (You may think Harris would deny such a conclusion, but in fact he admits that it is a consequence of his view, and accepts it.)
Even if you believe that in this case it would be a good thing for all of us to be devoured by aliens, it seems you must admit that the question “is it really a good thing?” makes sense. It at least is the case that someone can have reasonable doubts about this.
Another way to see the problem is this. Two people can disagree about the desirability of the aliens’ dinner plans, even if they agree that it would increase well-being. That is, they can agree with respect to the well-being involved, yet still disagree with respect to the goodness of the event. And that’s because “well-being” does not mean the same thing as “good.”
What Harris is actually doing in The Moral Landscape, then, is re-defining moral terms. His view that “questions about values… are really questions about the well-being of conscious creatures” depends on meaning something different by “values” than what is normally meant by that term. He is not talking about the same thing that most people are talking about when they discuss values.
Part 2 addresses the analogy Harris makes between ethics and medicine, an analogy he introduces in order to handle what he calls the value and persuasion problems.