I'll throw my hat in the ring here.

I've been in the hosting business for over 6 years. Needless to say, I'd compare cloud hosting with the shared hosting methodology of the late 1990's to early 2000's. Today however, Cloud Computing has become a much different animal. There are higher levels of security, performance and manageability that are defining what cloud computing truly should be and is becoming right now.

Anyway, to your questions;

1. OS: I really think the OS selection is based on the capabilities of the provider and their ability to support those needs with experts. In my mind, Cloud Computing should offer both Microsoft and Linux based operating systems.

2. Virtualization: This is a piece of the puzzle. Right now, VMWARE, Citrix and Parallels are the only companies providing what I'd say is an easy to deploy platform to offer a scalable and secure computing platform. In the future, the underlying virtualization technology will matter less when API's and customization become more prevalent. At this moment, I'd say that Citrix and VMWare will dominate for quite some time because of their financial capabilities and their general acceptance as reliable products. Although Microsoft and Google will have something to say about that.

3. Software: Id say any development platform should be built to live in a multi-tenant configuration and can easily scale across multiple processors.

4. Network: This is a big thing and the cloud most certainly should have more than 1 Tier-1 (Verizon,ATT, Level3) provider connected to it. As someone mentioned earlier, geo-diversity- or federated cloud- will build a truly resilient network for maximum uptime. Look for this from hosting.com in 2010.

5. Datacenter: Tier3 or better data center. Multiple carrier access, N+1 or better power and cooling. 24x7x365 support.

6. Other features: Well, API support, geographic load balancing, easy to use customer interface (Self Service).