Gratitude combines the awareness of unbidden blessings in our lives (gratefulness) with the ability to articulate this awareness to ourselves and to others (thanksgiving, with a lower-case "t"). In my own life, I have been uncommonly blessed to be born in a strong, loving family; to have had a great education; to have found my vocational calling; and to have had the opportunity to share my passions with others who care about them. It doesn't get much better than that.
Most often, gratitude is seen as a virtue because of its connection to generosity--an outward display of this inward "feeling". As I reflect more deeply, however, perhaps the most significant impact of an "attitude of gratitude" is in how it changes the one experiencing it.
To realize a moment of the sheer beauty in a November sunset, or to receive the unqualified forgiveness of someone you've unintentionally hurt, or to experience the joy of seeing someplace entirely new--these change the lens trough which we see the world, how we interact with others, and how we see ourselves in relation to others. In short, it changes for the better how we experience this world.
This is the true gift of Thanksgiving (with a capital "T" this time). Gratitude is a gift that can keep on giving to each of us all year long.