In Brave New World, John is an archetype: a "noble savage." Huxley bases him on Dryden's play The Conquest of Granada (1672). As such, the inhabitants of the Utopia look on a noble savage to validate their own society. Their thinking is such: if we can reform and civilize a savage into a gentleman, then our society is indeed a utopia.
John becomes a source of sentimentality for the Utopians: they pity him. They treat him as a mascot, a pink poodle, as a sinner in search of salvation, a prodigal son. He is everything they are not: born of woman, educated, ashamed. He is a kind of Christ figure, but instead of him redeeming their sins, they want to redeem his.
Mutapha Mond has conditioned the Utopians to believe that naturally born humans are base, evil, savage, and corrupt. Mond has conditioned them to believe that society is the defining, transformative aspect of identity. They do not believe in free-will; rather, they believe in "Identity, Stability, and Community." By adopting a savage into their ranks, all of Mond's teachings have been validated. When John partakes of soma and sex, the Utopians view it as if a cannibal were converting to Christianity. If their society can save this soul, then anything is possible.
Not only that, but the Utopians have been conditioned to consume. They literally want to consume John in a kind of parody of the Last Supper. They describe him as delicious, and they want him to partake of their soma, feelies, and orgy-porgy. They want to be entertained. They want gossip. John is a grand new media, a living technology for them to consume.
I wonder if, for the second question, you are referring to what is said in the first sentence of the chapter. There, John the Savage is said to be "delicious."
I think that there are two reasons why everyone wants to meet John. First, he is a complete novelty. This is a society where people want to be entertained continuously. Here is a new thing for them to look at.
Second, he is so scandalous. Here is a guy who single handedly brought down this important guy -- the Director. So people are just curious to see this guy who has had such a big impact -- he's a celebrity.