This post is as per beta version, there could be some changes with RTM version.
SharePoint 2013 includes new upgrade features, such as upgrade for service applications, a site health checker, and upgrade for site.
SharePoint 2013 does not support in-place upgrade for an existing environment. You must use the database-attach upgrade method to upgrade your databases to a new environment that is based on SharePoint 2013. Also, to provide more flexibility to farm administrators and site administrators, the upgrade process has changed to separate upgrade of the software and databases from upgrade of the sites.
A list of important facts along with known issues and solutions for SharePoint Foundation 2013.
Known issues and solutions for SharePoint Server 2013 and Microsoft SharePoint Designer 2013.
List of common issues and describes general principles to help you identify and address upgrade issues. After you identify and address the issues, you can resume upgrade. Learn how to address problems that may occur after you upgrade a database to SharePoint 2013.
When you upgrade a site collection to SharePoint 2013, errors can occasionally occur. Learn how to address problems that may occur after you upgrade a site to SharePoint 2013.
Before you start to upgrade from SharePoint 2010 Products to SharePoint 2013, make sure that your environment is functioning in a healthy state and clean up any content that you do not have to upgrade. Also take the time to remove or rearrange content. Make sure that your environment is in a healthy state, and delete unnecessary items before you upgrade to SharePoint 2013.
Upgrade the service applications (Business Connectivity Services, Managed Metadata, Secure Store, User Profiles, Search) to SharePoint 2013. When you upgrade from SharePoint 2010 Products to SharePoint 2013, you must use a database attach upgrade, which means that you upgrade only the content for your environment and not the configuration settings. After you have configured the SharePoint 2013 environment, and copied the content and service application databases, you can upgrade the service applications to SharePoint 2013.