The Spirit foreshadowed the vision which Nephi was to receive, and Nephi's previous understanding of his father's dream and this introduction by the Spirit were certainly influential as the vision unfolds to Nephi.
The vision begins simply, with the Spirit commanding that Nephi look. That simple mechanism shifted Nephi's focus from the personage who guided him to the vision itself, a technique which is repeated in verses 12 and 13 below.
Nephi sees the tree, and now adds his description. In some way the visual characteristics of the tree were capable of impressing its value upon the viewer. It is probable that some of the visual characteristics do not translate well in the telling. It is hard to imagine a white tree with white fruit being beautiful, but this one clearly was. It is likely that the whiteness of the tree is connected with the whiteness of light that is described around beings of power. Note the descriptions of white and light in the following verses (emphasis added):
Matthew 17:2
2 And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light.
3 Nephi 19:25
25 And it came to pass that Jesus blessed them as they did pray unto him; and his countenance did smile upon them, and the light of his countenance did shine upon them, and behold they were as white as the countenance and also the garments of Jesus; and behold the whiteness thereof did exceed all the whiteness, yea, even there could be nothing upon earth so white as the whiteness thereof.
Joseph Smith History 1:32
32 Not only was his robe exceedingly white, but his whole person was glorious beyond description, and his countenance truly like lightning. The room was exceedingly light, but not so very bright as immediately around his person. When I first looked upon him, I was afraid; but the fear soon left me.
The last statement by Joseph Smith is somewhat less clear than the other two, but I believe is describing the same phenomenon. The physical glory of the celestial on this earth translates into a visual experience which can only be described in terms of light and whiteness. It is clearly in this context that we should see the whiteness of the tree and the fruit, and thereby understand why it was immediately appealing to Lehi and Nephi - it spoke to their soul directly of the glory and joy of the celestial world.