Sup Jorge! You named the big ones. There are a bunch of others, as well. IMO, try to get your comparison down to just two. That's a reasonable set to dive deep on.
For a lighter overview, I think the big features are: team workflow, security, and CI/CD pipeline.
Team workflow is just how easy it is to get going / share with your team. IMO, ECR or GCR will have a natural edge here if you're already on their cloud. Tagging is important too, but I think everyone supports that.
Security is both the details of transport (SSL, etc), and whether your containers are getting scanned. Quay.io and Docker Hub both do security scanning for private repos. Quay has a slight edge in that public repos also get scanned thanks to Clair. I believe GCR and ECR lag behind here.
CI/CD pipeline is important because your registry becomes a big chunk of your build. This is what's going to really take time to investigate and dig into. You want to make sure it's easy to add hooks to git or w/e, and troubleshoot build issues (good logging, auditing, etc).
Full disclosure I work at CoreOS and with the Quay folks. That said, I also think they're constantly probing into cool frontiers. I think Clair changed registry security. The team's also started doing cool stuff for k8s users [1].
Lastly, I'm not totally sure on this last bit, but I think Docker Hub has a slight usability edge if you're on Docker EE (swarm).
Summarizing: I think big cloud vendors will naturally always lag a little behind. They'll make up for it with convenience if you're already on their cloud. Registries whose main purpose is to be a registry (like Quay) will naturally innovate a little faster.
[1] https://coreos.com/blog/quay-application-registry-for-kubern...