The new collaborator feature makes it easier to include stakeholders in the communication loop.
The feature I really think github is lacking right now is the ability to give selective permission to some sections of the repo (issue|wiki|code|releases). Right now the only way to share issues but not the code is to create a separate repository, which is cumbersome to do and maintain for every project.
I can't overstate how useful it would be to at least restrict write access to certain branches. Bitbucket has had it for a while now [1], so I'm surprised GitHub haven't followed suit.
I don't know if I would use branch restriction, it sounds like an anti pattern for the git philosophy (SVN file locks, anyone?). Why would I commit to someone else's branch, unintentionally? I think that a "gentlemen’s agreement" as your link states, is more than enough. Can you provide a use case where this feature is useful? Do you use it often in your workflow?
I can see it being used for autodeployments on a "stable" branch and no deployments on other develop branches, and the stable branch being write-restricted.
I'm very, very excited about this. The poor level of control over permissions in orgs were a big issue when we (LXQt) moved our development to Github. Look forward to trying it out.
The feature I really think github is lacking right now is the ability to give selective permission to some sections of the repo (issue|wiki|code|releases). Right now the only way to share issues but not the code is to create a separate repository, which is cumbersome to do and maintain for every project.