You don't give an excerpt in your question, so I will contain my answer to the first stanza of the poem, as representing its "opening lines."

We can certainly argue that the opening lines of this poem convey a sense of wonder and admiration. The phrase "still unravish'd bride of quietness" is a good example. The metaphor Keats uses here creates the impression of something that has never been touched, and yet which potentially contains wonders: it is waiting to be "ravish'd" and plundered for what it could yield.

Keats' reference to the "foster-child of silence and slow time," too, suggests awe and admiration. "Slow time," in particular, implies a long and quiet expanse of time beyond the poet's comprehension; the urn, in the speaker's mind, has been "fostered" by ages going back into ancient history. In its long life, the urn has also become host to "leaf-fring'd legend" which "haunts about" its shape. The use of the word "legend" seems to connect the urn to the great stories of the ancient past, by which it is haunted. The poet then mentions "deities and mortals," "men and gods," suggesting that the urn is a living memory of a time when these two types of beings were brought into contact with each other. Indeed, the phrase "Sylvan historian" personifies the urn as a historian of ancient times, who, through what is painted upon it, can speak to the poet of "slow time" long past.