I am considering making my closed source free D&D character generator redblade open source. What should I focus on to make my open source projects prosper?
To define success; When my now closed source project survives, with a nice community of users and contributors.
In that order.
To clarify that, every successful open source project requires outstanding leadership. Think Larry Wall and Perl, Linus Torvalds and Linux and so on. As much as open source is a community effort, it requires someone pushing the thing forward and doing so without fragmenting is discouraging the community.
It's also worth asking the question: how do you define "success"? You have a closed source project. What are you hoping to achieve by making it open source? Quality? Adoption? Contributors to the code base?
By prosper, do you mean having lots of users, or lots of users and developers? I'd guess the latter, so focusing on being developer friendly, some thoughts:
I think the key ingredient is that it has to be interesting to people who are willing and able to continue to develop it. I think the vast majority of open source projects are still probably single developer projects whose source code is available to all. That's not particularly bad -- as a user, it's nice to have access to the source code so I can understand how it works even if I never intend to modify it. If you're going to define success as a having a community of contributors, though, I think the primary factor is whether your intended users will likely have the ability to make useful modifications. Once you've passed that hurdle, then I think how your ability to manage the community and the inevitable conflict that will arise is the key ingredient in making it successful.