In reply to John Teague.

Standout issues that come to mind are:

1. Multiple users can't collaborate on a single PR unless they each have write access to each other's forks.

2. The decentralized nature of git actually hinders contribution in a project of WordPress' size. When you clone WordPress, you get the entire history which can be prohibitively large. We have more than 35,000 changesets and counting — that's a lot of history to download.

3. The tagging system in PRs and issues is pretty barebones. Especially when you consider the depth and breadth of the WordPress codebase. I don't have a current count, but I think we have something like ~300k lines of code spread across 50+ components on Core Trac right now. This is almost unmanageable as-is, and yet with self-hosted Trac and SVN we have a lot more control over availability of tooling because we can create our own tools as we continue to scale. Moving to GitHub largely hinders the ability to scale from an organizational perspective.