On the surface, this poem seems to be about the nature of art. Keats says profound things about the nature of the plastic, visual arts, and their status of being outside of time.
Heard melodies are sweet, but those unheard
Are sweeter; therefore, ye soft pipes, play on;
Not to the sensual ear, but, more endear'd,
Pipe to the spirit ditties of no tone: (lines 11-14)
How strange that seems! To not enjoy the actual sound of the music, but to contemplate the eternal depiction of it, is sweeter to the poet. This sort of denied gratification, this restraint as represented by the painting on the urn ("Thou still unravish'd bride of quietness") would appeal to the philosopher or the poet, or perhaps the connoisseur, but to the casual observer it seems a bit illogical. Why is the fact that the piper can play on forever, metaphorically, somehow better than hearing a real piper? Keats doesn't mean this literally, I believe; he is showing us the nature of the art, and its beauties, but also the importance of immediate experience. Yes, the urn is beautiful, but the scene of the running maiden, the piper, the lover, the empty town, etc., is mute, and separate, and outside of our time. It can teach us about beauty and truth, but it is outside of our temporal life, because it has endured so long and will endure long after the people looking at it are gone.
Thou, silent form, dost tease us out of thought
As doth eternity: Cold Pastoral!
When old age shall this generation waste,
Thou shalt remain, in midst of other woe
So does Keats love the vase, or not? He certainly appreciates its beauty, but he is also warning against the lures and temptations of art (especially material, visual art). Yes, we can learn beauty and truth from this vase, he is saying, but it is not the same as living the life it depicts; it is not hearing the pipes, running like a "leaf-fringed legend", or attending the altar with the townspeople. It is merely a depiction of what the beauty of the world can be, but it is not the attainment of it. This is expressed in the second stanza
Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare;
Bold Lover, never, never canst thou kiss
Though winning near the goal—yet, do not
She cannot fade, though thou hast not thy bliss,
For ever wilt thou love, and she be fair!
This set of lines shows the beauty of the scene, but also the emptiness of the depiction of art. It is important, I think he is saying, to remember that this is not any kind of reality; the depiction of the events shows us the beauty, but not the reality of the moments (because it is outside of time). The lover can never kiss; we can look at this bit of truth and beauty, but the ultimate goal can never be attained.
This poem, while beautiful in prosody and content, is really a rather solemn meditation on the brevity of life and the limits of artistic experience. Though the ending lines seem very final and profound, "Beauty is Truth, Truth Beauty,"—that is all/Ye know on earth, and all ye need to know" it is clear that this vase, which is a "friend to man", can only teach us about beauty and truth. This is not the same as living beauty and truth, and "knowing" is not everything. It's a celebration of the art, but also a mindfulness of its limitations, that makes this poem bittersweet.