tl;dr:Agile is not achieving a sustainable speed in an optimal direction; "it is being able to change both speed and direction, and to do so easily. It is the second derivative of position with respect to time, not just the first: agility relates both to velocity and to the ability to change it, revising direction and pace as circumstances demand."
tl;dr:"Technical debt is not the cost of repaying the debt: it is the cost of owning the debt. These are not the same." It's also not necessarily a negative thing. Kevlin feels the metaphor is commonly misused.