In case it helps, here's a rough list of the thoughts that have come to mind:

• Simplicity is usually best with voting systems. It may be worth looking at a reddit style up/down system for popularity. With importance you probably want high/mid/low. If you track the 'importance profile' of a user, you could use that to promote projects to their attention that other users with similar profiles find important. Also, in all these rankings it should be clear to the user exactly what metric is being used.

• Make use of the wisdom of crowds by getting users to evaluate projects/tasks/comments for things like difficulty, relevance, utility, marginal utility - along the lines of this xkcd comic.

• It seems to me that good open source management tool should direct attention to the right places. Having inbuilt problem flags that users can activate to have the problem brought to the attention of someone who can solve it seems like a good idea.

• Skill matching. Have detailed skill profiles for users and have required skills flagged up for tasks.

• Could try breaking projects up into a set of tasks, sub-tasks and next actions a-la Getting Things Done

• Duolingo provides free language courses. They plan to make this financially viable by crowd sourcing translations from their students. Perhaps a similar thing could be implemented - maybe by getting university students involved.

• Gamification across a broad range of possible tasks. Give points for things like participation, research, providing information. While rewarding programmers for coding is good, we should seek to reward anything that lowers the activation energy of a task for someone else.

• Keep a portfolio of work that each user has completed in a format that is easy for them to access, customize and print out and reference in job applications.

• Encourage networking between users with similar skills, areas of interest and the like. This would provide a benefit to being part of the community.

• You could have a Patreon like pledging system where people pledge a small amount to projects they consider important. When the project reaches a milestone the contributors then get rewarded a portion of the pledge.