My guess is things like alcohol and porn, things that are not intrinsically illegal but are still legal hot potatoes of compliance. An online alcohol vendor that had an "age check" that was easy to game ("yes, I'm 21") would be easy to set up and make a lot of money, but a lot of laws would leave Stripe legally in bad shape when the hammer came down, but the PR could be an even bigger disaster.

In addition to the obvious -- bootleg cigarettes from reservations and booze.