For women, the standard of what's sexy or not is entirely dependent on the shape and style of the woman, not to mention the clothes that the jewelry is going with. A little goth girl is going to look smashing in a black velvet choker and numerous bangles and bracelets, mismatched, topped off with a pair of dangly earrings and maybe some higher ear piercings. On the other hand, a club girl is going to probably find a simple high-line necklace in shiny glass more her style, with earrings that are simple enough to match without getting caught in her hair on the dance floor.
I make a point of not wearing a lot of jewelry, although I find a certain sophistication and grace with wearing a pair of small simple hoop earrings. When I'm going out, if I don't have anything that matches my outfit, I go without or else wear just a simple gold or silver chain. (That's not counting that my husband thinks the sexiest jewelry I wear is my wedding ring.)
For men, again, it depends a lot on their personal style and body type. A furry man should not wear a lot of necklaces, as a skinny man should probably avoid overly large watches. I enjoy men who wear simple earrings (usually hoops, or captive-bead rings), and a ring or two. Why? Because it's interesting. Gaudy rings beg the question of "Why are you wearing those?", and that's always a great way to start a conversation. When I notice people's jewelry (especially men's), I tend to ask myself why they're wearing them. If someone is wearing a little cross or other religious piece, that's an interesting thing to me. If they're wearing a little fertility symbol (like the claws from the 70s), that's interesting and humorous to boot. Wide band watches look good on people with tattoos, and simple yet substantial watches or bracelets look good on men with a medium or large build.
I'm one of the few folks that finds body jewelry more interesting than sexy, probably because I've been around it for so long. Belly button piercings, although initially visually attractive, just bring back horror stories from friends about getting the ring caught on all sorts of things during romantic interludes. Le Brets (a piercing in the divot below the center of the lip) can be very sexy, depending on the jewelry (gaudy is irritating, simple is intriguing), as can a piercing through the bottom of the lip in the center (creates a kind of pouting effect). Nipple piercings are done more for personal pleasure than for aesthetics, so I don't count them as "pretty" jewelry.
Then again, people that enjoy body piercing aren't normally concerned with what's "sexy" so much as what's "risque". For my part, I enjoy my jewelry because I know what it means to me, and I could care less what anyone else thinks. My husband fell in love with me with my septum ring in, so I think he's probably okay with it on the "sexy-meter".
Only those that attempt the absurd, achieve the impossible.