Hard drive is a commodity. Whether it is IDE, EIDE or SATA, you buy the right one and you can plug it directly into your computer.

A commodity is a good for which there is demand, but which is supplied without qualitative differentiation across a market. Commodities are substances that come out of the earth and maintain roughly a universal price
Quote - Wikipedia

Cloud Storage is becoming a commodity. The pricing is becoming similar with the interface becoming similar too across cloud storage vendors.
There are three layers in the cloud storage stack:

1. cloud storage software facing the end user;
2. service providers providing the services
3. the cloud storage vendor providing the solution to the service providers.

On the user front, more and more cloud storage software are available supporting more and more cloud storage vendors. For example, in 2008, Gladinet supported SkyDrive, Google Docs, Google Picasa and Amazon S3. In 2010, the list expands to include AT&T Synaptic Storage, Box.net, EMC Atmos Online, FTP, Google Docs, Google Apps, Google Storage For Developers, Mezeo, Nirvanix, Peer1 CloudOne, Windows Azure, WebDav and more. This trend enables the consumers to pick and choose which cloud storage services they need.

On the service providers front (service providers are the biggest cloud storage service providers),  they may not all create cloud storage services themselves. For example, AT&T and Peer1 are using EMC Atmos. Verizon and Planet are using Nirvanix. As time goes on, we will see the same provider using multiple backend solutions to satisfy different need. Also when service providers merge, the merged company may be using backend solutions from multiple vendors.

On the cloud storage vendor front, more and more are conforming to the Amazon S3 API. We saw Google Storage for Developers, Eucalyptus, Dunkel and Mezeo all creating S3 compatible APIs for their cloud storage solutions. On the other hand, Rackspace is pushing the OpenStack project. All are trying to create a unified interface.

All these are turning cloud storage into a commodity.