What, according to Aristotle, constitutes a life well-lived, and how does his conception of this relate to his conception of virtue and his ideal of virtuous action that avoids both "excess" and "deficiency" but rather that realizes "a mean"?
Aristotle concluded, although this is simplifying matters greatly, that a "life well-lived" was the life of contemplation that led to an understanding of the principles which govern the operation of the universe.
That is quite a mouthful.
This principle was used to reduce decision-making down to a technical degree, from which someone could theoretically achieve the "proper" virtuous decision by rationale choice. This is not in and of itself a value judgment. For example, bravery could be thought of as a virtue. Cowards face no challenges, and have a deficiency of this virtue. Those with an excess of bravery are often unable to correct gauge battles worth fighting-- and battles they are sure to lose.
In between all of these extremes, the general landscape of virtuous action could be found somewhere in the middle. However, Aristotle was careful to avoid imposing absolute virtue. Rather, the mean that exists between excess and deficiency is different for each person, and unique to their circumstances.