I suspect that a lot of people's flavor of productive might turn on "artist" or "writer" where very few will be interested in "coal mine equipment operator", or "customer service person for DELL." For the foreseeable future there will always be jobs people don't want to do that machines aren't capable of taking over.

I agree with this. What will be interesting though is to see if the wages of those jobs will increase because people won't want to do them, or if wages will decrease because there will be so many more people that aren't employed, and there may be a certain prestige to being employed to do any work at all.

I suspect that no matter how you slice things, there will always be people who don't qualify for UBI (probably anybody without citizenship or a green card), and that those people will end up working many of those jobs for low wages.

Yes, and it's a very good thing that a UBI system still allows the "coal mine equipment operator" to be paid for their work on top of the UBI.