I love S3, and we use it all over the place at transloadit.

I realized my comment sounded overly negative, so I added a clarification to my comment: Our goal is to bring resumable file uploads to the entire planet, S3 or any other proprietary protocol should not be the base for that.

What's your opinion on Swift (http://www.openstack.org/software/openstack-storage/)? It's basically an open standard for "an object store with compatibility to, and the same guarantees of, S3." Used in, for example, Rackspace Cloud Files.