The tone is the way the author feels about his subject, which is sometimes different than the mood (which is how he/she makes the reader feel). In this case, it seems that the tone and mood are the same.
Wordsworth, one of the first-generation Romantic poets focuses strongly (as is a characteristic of Romantic poetry) on nature: he praises it and finds in it a pattern that mirrors the phases of a person's life. (He also must consider the intellect and one's memory in this process.) In "Lines Composed a Few Miles Above Tintern Abbey," Wordsworth is contemplating the following:
Is the adult perspective on nature (and on life in general) a triumph or a loss?
The phases of life as they are tied to nature are childhood- adolescence and adulthood. In childhood-adolescence, the interaction with nature is innate—the response is not based on the intellect but upon sensory experiences. The freedom of youth is like the freedom of the deer, streams—nature itself—
…like a roe
I bounded o'er the mountains, by the sides
Of the deep rivers, and the lonely streams,
Wherever nature led…
…For nature then
(The coarser pleasures of my boyish days,
And their glad animal movements all gone by,)
To me was all in all.
However, in adulthood, reason has stepped in: the intellect is engaged to ponder larger questions about one's existence—and not only the experience of the earlier stages (through memories), but also the cessation of these carefree, natural times that are lost as one grows older.
In these three stages, the poet is left to wonder what is gained and what is lost through the transitions to the final phase. Certainly there are benefits in childhood as opposed to adulthood (and vice versa), but is adulthood better than childhood in the way nature (and life) is perceived?
With all the references to nature, it can be inferred that Wordsworth looks to nature for these answers. In the end, it seems that the speaker believes that for the things of youth in nature that he has lost, he has been compensated by knowledge that comes with age. His intellect has allowed him to look at nature differently: not taking it for granted—
That time is past,
And all its aching joys are now no more,
And all its dizzy raptures. Not for this
Faint I, nor mourn nor murmur: other gifts
Have followed, for such loss, I would believe,
In his contemplation of who he was and the abandon with which he enjoyed all that nature had to offer, he has learned that it is far greater to be older and know how to appreciate nature.
For I have learned
To look on nature, not as in the hour
Of thoughtless youth...
The speaker has learned that even in the middle of humanity (with its "sad music") that nature still has power—there is a...
...presence that disturbs me with the joy
of elevated thoughts.
This refers to the knowledge of what adulthood has brought to him—his ability to fully recognize and value the enormous impact of things like...
And the round ocean, and the living air,
and the blue sky,
and the mind of man...
The awareness of nature that exists throughout every aspect of life is worth what the author has left behind in his youth. He notes:
The guide, the guardian of my heart, and soul
Of all my moral being.
The structure of identifying the phases of nature to coincide with the phases of life, is effective in conveying Wordsworth's tone—youth brings freedom; adulthood offers true appreciation.