Here's a list I just recently put together of the ones I've had selected if it's any help: https://medium.com/@benulansey/list/boosted-by-medium-2149c832a405

I think Medium's guidelines have been a fairly accurate descriptor of what they're looking for when I consider my own pieces that have been Boosted. It's not easy to get selected, though. Of the many pieces I've written that I feel deserve more attention than they got, only a few of them were boosted. And in only one case did the boost make a true difference for me. So it's not quite all it's cracked up to be. Great piece.