What distinguishes a superstar from a mere star?

Titles held and win/loss records.

I'm gonna go with notoriety outside one's field of endeavour. So if people who don't follow the industry you're in still no who you are, you're a superstar.
You also get invitations to do crossover appearances on TV shows, or in movies, national commercials that don't air inside your field, etc.

Jason has it I think. A person who transcends their field. When they walk in a room you know they're somebody, just by their presence. Michael Jordan comes to mind as the epitome. Literally the most famous person on earth in the 90s. People who didn't even know anything about basketball knew Mike, globally.

Even people who aren't KISS fans know who Gene Simmons is.

Whether he's notable or notorious, he has branded himself well.

I know Gene Simmons, but who or what is KISS? Is it an acronym for something?

This, 100%.

I think of Tony Hawk or Wayne Gretzky - I have no interest in either sport, but I know exactly who those guys are and what sports they're famous for being involved with. Same with Lance Armstrong.

Also people who are so famous they only need one name (Madonna, Prince, etc). Arnold Schwarzenegger became known simply as Arnie so that makes him a Superstar. Sylvester Stallone was referred to as just Stallone for years and you can still call him that and everyone will immediately know who he is. Lady Gaga is just 'Gaga' now and everyone knows who she is so I guess she also counts as a Superstar.