I think there are some great snakes to keep at the beginners level. And I don't know what "qualifies" someone as ready to keep another species. I'll use bloods as an example. On different websites like NERD, they state that it is an intermediate level snake. For people that have successfully kept other species and can recognize certain problems with their set up or husbandry or the snake itself.
I think if you feel confident enough with keeping a certain species, and you feel ready to take on the financial commitment (not just price, but feeding and any health issues), Go for it.

Do your research, ask questions, talk to breeders and keepers about whatever species your really interested in and get a feel for behavior, husbandry, feeding, pooping, shedding and common problems.