Windows Server 2012 is no doubt the new cloud OS and it's super combination with System Center 2012 truly qualifies it to be the most powerful server operating system in the world today. In 2012 release there has been a change in Product licensing and it has been simplified keeping in view the ease of our customers.

A few queries which you might have are enlisted below:

What is new in Windows Server 2012?
Windows Server 2012 brings our company’s experience building and operating public clouds to the server platform for private clouds. The new licensing and packaging makes it easier to manage workloads in highly virtualized public and private cloud environments. Windows Server 2012 will move to a consistent licensing model and will have common features enabling the reduction of editions. These include Two editions, Standard and Datacenter. Single licenses that cover up to two physical processors. Editions differentiated by virtualization rights only (two for Standard; unlimited for Datacenter).

What is the difference between Windows Server 2012 Standard edition and Windows Server 2012 Datacenter edition?
Both Standard and Datacenter editions provide the same set of features; the only thing that differentiates the editions is the number of Virtual Machines (VMs). A Standard edition license will entitle you to run up to two VMs on up to two processors (subject to the VM use rights outlined in the Product Use Rights document). A Datacenter edition license will entitle you to run an unlimited number of VMs on up to two processors.

Will Windows Server 2012 Standard edition have all of the same features as Windows Server 2012 Datacenter edition?
Yes. All features that are available in the Datacenter edition are also available in the Standard edition, including high availability features like failover clustering. The only difference between the two editions will be virtualization rights.

How do I determine which Windows Server 2012 edition  is right for me?
Since there is feature parity between Windows Server 2012 Standard and Datacenter editions, your decision will be based solely on your virtualization strategy as virtualization rights become the only differentiator between editions. If your strategy calls for a highly virtualized environment, Datacenter edition will provide you with optimum flexibility since it allows for unlimited virtualization. If you do not plan on a highly virtualized environment at this time, Standard edition is the right product for your needs. If you purchase Standard edition today but find that you need more capacity in the future you will have two options to expand the virtualization capacity of your licensed server: 1. Purchase additional Standard edition licenses and assign them to the same physical server giving you the rights to run additional instances of Windows Server, or 2. If you have Software Assurance on your Standard license you can purchase a Software Assurance Step-Up and migrate to a Datacenter edition license on that server If you are running a highly virtualized environment, management may also be a need for you. You should consider purchasing System Center 2012 with Windows Server 2012 together in the Core Infrastructure Suite, which is available inside or outside of an Enrollment for Core Infrastructure (ECI) agreement.

Is Enterprise edition going away as part of Windows Server 2012 and why?
Yes. Enterprise edition will be retired as part of the Windows Server 2012 release. Windows Server 2012 Standard edition will include all the premium features previously included in Enterprise edition in the past and the price to purchase the rights to 4 instances of Windows Server 2012 will actually be less expensive than the price of Windows Server 2008 R2 Enterprise edition today. Due to these changes, we have been able to simplify the product lineup while reducing the price per instance of Windows Server for these customers.

If I have Software Assurance in place at the time that Windows Server 2012 is generally available, when will my grant be reflected in my licensing records?
While you will immediately be able to use the new product and will own the perpetual rights to the edition of Windows Server 2012 that you are transitioning to, your grant will not be reflected in the license position database until you renew your agreement and purchase Software Assurance on the granted licenses. If you do not renew your Software Assurance, your license position will not be reflected in the license position database; however the fact that you had Software Assurance in place at the time of Windows Server 2012 General Availability will establish your proof of license for the new product.

If I have Datacenter edition with Software Assurance when Windows Server 2012 is released, which edition will I be entitled to receive?
If you have Software Assurance on Datacenter edition, you will be entitled to Windows Server 2012 Datacenter edition. Today, a Datacenter license covers up to 1 processor. A Windows Server 2012 Datacenter license will cover up to 2 processors. So for every two current Datacenter licenses with Software Assurance, you will receive one Windows Server 2012 Datacenter edition license.

If I have Enterprise edition with Software Assurance when Windows Server 2012 is released, which edition will I be entitled to receive?
If you have Software Assurance on Enterprise edition, you will be entitled to receive two Standard edition licenses for each Enterprise edition license you have. If you are considering moving to a more highly virtualized or private cloud environment, you should consider taking advantage of the Software Assurance Step-Up* benefit to upgrade to Datacenter edition prior to the Windows Server 2012 general availability since the Step-Ups from Enterprise edition to Datacenter edition will be removed from the price lists at that time.
* If you have an Enterprise Agreement and already have an SA Step-Up from Enterprise edition to Datacenter edition on your Channel Price Sheet (CPS), you will be able to Step-Up from Enterprise edition to Datacenter edition until the end of your enrollment.

If I have Standard edition with Software Assurance when Windows Server 2012 is released, which edition will I be entitled to receive?
If you have Software Assurance on Standard edition today, you will be entitled to receive Standard edition at the general availability of Windows Server 2012. You will receive one Windows Server 2012 Standard edition license for each Standard edition license you have.

Can I use my Windows Server 2008 CAL to access  Windows Server 2012?
No. The CAL accessing the instance of Windows Server must be equivalent or higher in version than the server being accessed. You will need a Windows Server 2012 CAL to access a Windows Server 2012 instance.

FOR MORE QUERIES / SCENARIOS PLEASE FEEL FREE TO VISIT THE FOLLOWING LINKS, THE ABOVE FAQs ARE JUST THE MOST FREQUENT ONES:

http://download.microsoft.com/download/4/D/B/4DB352D1-C610-466A-9AAF-EEF4F4CFFF27/WS2012_Licensing-Pricing_FAQ.pdf