Here my thoughts.
1. Charge back model is not clear.
2. In case discrepancy of SLA's . Hosting providers doesn't gurantee Data security.
3. Due to Multi Tenancy and shared resources, Their could performance impact.
4. Iaas works over Internet, There should be dedicated bandwidth on system access and redundancy to be planned in case Internet service provider is down . Changes in architecture diagram , more approvals.
5. VPN connectivity is required Iaas services.- Architectural impact
6. SSO will be issue , due to multiple sign on's..- Architectural impact
7.Security issue's - data over the internet will be concern ..- Architectural impact
8. how do we monitor our servers ? In case of failure of hardware..- Architectural impact
9. Iaas providers should well defined the bounderies of access. In the sense hosting providers are located in North America. what are the chances of people across the global.
10. Scalablity of servers aross locations , geo
11. License of OS, Applications.
12. Data migrations between cloud , How does servers interact with different cloud ,

Interoperability

The decision to build a private cloud then is less about the investment and more about a business model decision, weighing up on cost/benefit and risk analysis.

How does the prospective IAAS client measure the benefit and upside, how do they pre-estimate the increased agility of IT, both technically and financially, that they will gain.

Many would be willing to pay more over time for a cloud solution if they get demonstrate a measurable business upside of an order of magnitude or competitve advantage through gained agility.

The challenge in IAAS is there are not enough cases or models to prove this. Finding the pionneers or the early adopters at this time is key. The laggards and slower adopters will come when they feel comfortable the risk is low and the benefits strong.

Identifying the easiest areas of the business that will benefit from IAAS such as web hosted platforms will help.

Challenges to overcome include facts such as the pricing models varying from provider to provider which makes it difficult for organisations considering a move to IaaS to compare providers against each other. The varying usage rates of different environments also work against organisations trying to compare prices.

Amazon Web Services appears as the most successful IaaS provider to date. Amazon has huge server farms that support the Amazon websites. It now offers the skills and infrastructure from providing their own services for hire to other organisations.

I think it's a combination of being scared and knowing how to utilize the current offerings to cater to its requirements. Organizations at this point have not spent much time on defining their infrastructure as it pertains to cloud computing. They are still pretty focused on in-house application infrastructure.
The keys here are to understand your infrastructure needs, identify what can be migrated to the cloud, examine TCO for that application or service and determine whether the cloud offers scalability, lower operating costs, any type of process automation etc.
Companies are starting to understand they need a platform, then infrastructure, application and then services to complement. IaaS can also refer to the services around cloud computing as well

Source: www.eval-source.com/blog