Integrate some kind of donation framework into Drupal.org to allow people to donate to maintainers of projects to help show their appreciation.
Move from forum discussions to Q&A format, with ability to vote up/down comments. Ability to seamlessly move from a support request to issue and back would be hugely beneficial.
Drupal.org is a bit painful to browse on a mobile device. It'd be cool if it used responsive design to show off what Drupal can do!
If commits reference a given issue, cross-post them to the issue so they're visible in the comment stream.
Before posting new projects, issues, forum topics, etc. show user a list of things with similar titles/descriptions and prompt user to confirm their submission isn't a duplicate.
In addition to running patches against automated tests, test them for performance as well so regressions are caught before commit!
Move job postings out of groups.drupal.org and onto drupal.org, and improve the listings with ties back to companies and locations + featured listings to help fund d.o development.
In addition to the community-managed documentation, the documentation team also wants to have a more curated documentation area, where we can have better control over the quality of the docs, as well as of what merits inclusion.
Provide the ability to create first-class forks / sandboxes of parent projects and perform pull requests, tightly integrated with the canonical project/repository.
Show upcoming events in your area, show what events people are going to on their profile, show a map of events around the world, etc.
Allow individual issues to be turned into Git repositories, which allows everyone working on an issue to collaborate in one spot.
Provide an alternate project_issue view for maintainers so that they can make bulk changes to things in their queue (e.g. re-componentize, tag, apply status changes, etc.)