> I think most of these questions are first and most importantly _hardware_, _money_ and _physics_ questions.

Actually, money (more accurately, cost) is a constraint instead of a resource. S3 is known for its low cost, and S3 can easily dote out a 70% discount to its large customers and still make a profit. So, an interesting question is how to build an low-cost object store/

> interconnecting _a lot_ of the fastest available storage

"A lot of" leads to real challenges. Sooner or later you'll find that managing metadata will become a tough challenge. Case in point, Open-source systems often use systems like Zookeeper or etcd or single-node name server with hot standby for metadata management, which certainly won't be able to handle the scale of S3.