Originally, MS will deliver Exchange 2013 RTM Cumulative Update 1 (CU1) by the end of this quarter. Unfortunately, we are not going to meet that goal. We know that many of you will be disappointed as a result of this statement. We understand your pain, however, the decision to delay is due to an issue we found in our final test pass coupled with feedback from members within our Technology Adoption Program community.
Specifically, we found an issue with Exchange 2010 coexistence. The issue actually had an easy workaround, but we made a decision; instead of burdening you with a configuration change on all of your Exchange 2010 Client Access servers, we decided to take a code change in Exchange 2013 and solve the problem so that you will not have to make any additional configuration changes. Given that the goal of CU1 is to enable coexistence with legacy versions of Exchange, we felt this was the right decision; after all, we want to ensure that your upgrade to Exchange 2013 and your coexistence period goes as smooth as possible.
As previously mentioned, Exchange 2013’s update strategy is different from previous releases; we are uncoupling security updates and reducing the number of updates we release. In addition to those changes, we will continue to evaluate issues as they are identified during development (even during the final test pass) and if we determine that the vast majority of on-premises customers are affected, we will do everything we can to mitigate the issue prior to release, even if that means delaying the release.
I have found these 2 videos on Tech-Net
The first video shows what is the new features in exchange 2013 and you can see the video here
The second video talks about:
1- New architecture of exchange 2013
2- High availability
3- Co-existence with exchange 2007 and exchange 2010
4- Upgrade path from exchange 2007 and exchange 2010
You can check the second video here