What seems to be the basis for their attraction to each other?
Romeo and Juliet's initial attraction seems to be founded on appearances. Before Romeo ever speaks to Juliet, he says,
O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
As a rich jewel in an Ethiope's ear --
Beauty too rich for use, for Earth too dear. (1.5.51-54)
He is immediately affected by Juliet's beauty. He says that she "burn[s] bright[er]" than a torch, and he compares her to a "rich jewel" being worn by night. He even says that her beauty is actually too much for the Earth to deserve. Romeo goes on and on about how beautiful Juliet is, and so it seems clear that this is what first attracts him to her.
The first time Romeo speaks to Juliet is to ask to kiss her hand. They speak very briefly and then they actually do kiss. Juliet's nurse pulls her away, and Juliet asks her to find out Romeo's name. When the nurse is away, Juliet says, "If he be married, / My grave is like to be my wedding bed" (1.5.148-149). Then, when she learns that he is a Montague, she laments, "My only love sprung from my only hate!" (1.5.152). In other words, she now feels herself to be in love, and considering she has only had one verbal interaction and one kiss with Romeo, her love for him must be, mostly at least, based on appearances at this point.