I think he is struck by her singing for, and by, herself, as a pure expression of beauty - as a perfect and lovely moment in itself - and as "the spontaneous overflow of powerful feeling" which Wordsworth saw as central to poetry and art - as well as it providing him with - as "The Daffodils" do in the poem of that name - a powerful emotional experience to reflect on in tranquility - another wellspring of poetry for him.
His respectful reaction to her, and the moment - is illustrated by his care that she not be disturbed - "stop here, or gently pass". There is an image of bounty and refreshment referring to her song -
O listen! for the Vale profound
Is overflowing with the sound.
and an echo, perhaps, of the 23rd Psalm?:
"Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me; thy rod and thy staff they comfort me.
Thou preparest a table before me in the presence of mine enemies: thou anointest my head with oil; my cup runneth over."
Here is a long and interesting critical essay, including a consideration of your poem:
" 7 November 1805, Dorothy Wordsworth wrote to a correspondent from Patterdale in the Lake District, enclosing a transcription of a poem that her brother William had written two days before. She explained that "The Solitary Reaper" had been "suggested by a very beautiful passage in a Journal of a Tour among the Highlands, by Thomas Wilkinson."(12) On the poem's first publication two years later, Wordsworth gave the same explanation in a note, adding that its last line was taken verbatim from Wilkinson. The source passage reads as follows: "Passed by a Female who was reaping alone, she sung in Erse [Highland Gaelic] as she bended over her sickle, the sweetest human voice I ever heard. Her strains were tenderly melancholy, and felt delicious long after they were heard no more."(13)
William and Dorothy had toured the Highlands of Scotland two years previously, and Stephen Gill is no doubt correct in saying that if Wordsworth had not himself "seen isolated figures set against the immensity of the Highland landscape the poem would not have been written."(14) But Gill does not speculate on why this particular passage had such an impact on the poet. In the poem's opening stanza, the reaper's isolation is repeatedly emphasized ("solitary," "single," "by herself," "alone"), an auditor is directly addressed, and his/her attention insistently drawn to this figure ("Behold her," "Stop here," "Listen"). At first sight, this might suggest that the particular attraction of the Wilkinson passage was owing to Wordsworth's longstanding interest in pathetic subjects and their focalization. But there is a crucial difference between "The Solitary Reaper" and such poems as "The Ruined Cottage" and "The Thorn," which also center on solitary females in particularized natural settings and have narrators who directly address an auditor. These poems are calculated mixtures of lyrical and narrative elements; but there is little of the latter element in "The Solitary Reaper." And since the speaker does not know the language in which the maiden is singing, he cannot apprehend the "theme" of her song, cannot determine what story, if any, it tells.........."
It continues here:
http://www.findarticles.com/p/articles/mi_m2220/is_n2_v38/ai_18405922/pg_2
This should give you enough to be going on with!