Microsoft has posted a comprehensive list of new features and functionality in SharePoint 2013 on their site, for developers, contents of the page below. Hope it helps. On a side note, the client-side object model (CSOM) is going to be vital in the newer iteration of SharePoint. As residing on an Office 365 dedicated environment for a while now, I'd say, getting away from most of the custom server-side development submitted through MSOCAF, will be a complete blessing, in favor of the quickly deployed client-side components.
Cloud App Model
SharePoint 2013 introduces a Cloud App Model that enables you to create apps. Apps for SharePoint are self-contained pieces of functionality that extend the capabilities of a SharePoint website. An app may include SharePoint components such as lists, workflows, and site pages, but it can also surface a remote web application and remote data in SharePoint. An app has few or no dependencies on any other software on the device or platform where it is installed, other than what is built into the platform. This characteristic enables apps to be installed simply and uninstalled cleanly. Apps have no custom code that runs on the SharePoint servers. Instead, all custom logic moves "up" to the cloud or "down" to client computers. Additionally, SharePoint 2013 introduces an innovative delivery model for apps for SharePoint that includes components like the SharePoint Store and the App Catalog.
Familiar programming model using web standards
The current release reflects enormous strides in optimizing the existing development tools like Visual Studio and SharePoint Designer, in addition to providing the release of newly developed web-based tool "Napa" Office 365 Development Tools for developing apps. The new unified project system in Visual Studio lets you develop apps for SharePoint, apps for Office, apps for SharePoint that include apps for Office, or apps for Office that are hosted by SharePoint. In addition to the SharePoint project templates that were provided in earlier versions, Visual Studio 2012 now includes a new app project template in the Apps folder named Apps for SharePoint 2013. Several new properties have been added to the Properties window and Properties pages to support app for SharePoint projects. Other improvements include full support for development against the Cloud App Model, including OData and OAuth support, and full support for development against the Workflow Manager Client 1.0 platform.
Core Platform Enhancements
On a broader scale, SharePoint 2013 has been improved and enhanced to support the new cloud-based architecture and app-driven development framework. From the SharePoint APIs at the lowest level to connectivity to social media integration, SharePoint 2013 is designed and executed to support a rich application development experience. In addition to the use of Representational State Transfer (REST) endpoints for web services, there is a broad new API for both server and client development. Remote event receivers and now supported in addition to client-side rendering.
With SharePoint 2013, you can combine Windows Phone 7 applications with on-premises SharePoint services and applications, or with remote SharePoint services and applications that run in the cloud (such as those that use SharePoint Online), to create powerful applications that extend functionality beyond the traditional desktop or laptop and into a truly portable and much more accessible environment. The new mobility features in SharePoint 2013 are built on existing Microsoft tools and technologies, such as SharePoint, Windows Phone 7, Visual Studio, and Microsoft Silverlight. You can create SharePoint-powered mobile applications for Windows Phone using the new SharePoint phone application wizard template in Visual Studio, which lets you create simple list-based mobile applications. You can integrate new features introduced in SharePoint 2013, such as the Geolocation field type and “push" notifications from SharePoint Server, into your mobile applications.
Social and Collaboration
New and improved social and collaboration features make it easy for users to communicate and to stay engaged and informed. The improved My Site social feed helps users keep up to date with the people and content that they care about. The new Community Site feature provides a rich community experience that lets users easily find and share information and find people who have similar interests.
Search functionality in SharePoint 2013 includes several enhancements, custom content processing with the Content Enrichment web service, and a new framework for presenting search result types. Additionally, there have been significant enhancements made to the keyword query language (KQL).
Workflow Manager Client 1.0 is a redesigned workflow infrastructure that is built on Windows Workflow Foundation 4 and brings new power and flexibility to workflow authoring in SharePoint 2013. A fully declarative authoring environment enables information workers to use SharePoint Designer 2013 to author powerful workflows, and a new set of Visual Studio 2012 workflow project templates let developers access more sophisticated features like custom actions. Perhaps most importantly, Workflow Manager Client 1.0 is fully integrated with the model for apps for SharePoint. In addition, workflows execute in the cloud, not in SharePoint, which provides enormous flexibility in designing workflow-based apps for SharePoint.
Enterprise Content Management
Business Connectivity Services
Business Connectivity Services (BCS) enables SharePoint to access data from external data systems such as SAP, ERP, and CRM, in addition to other data-driven applications that are exposed through WCF services or OData endpoints. BCS in SharePoint 2013 has been improved and enhanced in many ways, including OData connectivity, external events, external data in apps, filtering and sorting, support for REST, and others.
SharePoint Server 2013 includes several services for working with data in your SharePoint sites. New for SharePoint is the Machine Translation Service, which translates sites, documents, and streams for multilingual support. SharePoint Server 2013 also includes Access Services and a new data access model. For converting files and streams to other formats, SharePoint Server 2013 has Word Automation Services and PowerPoint Automation Services (a new feature for SharePoint). SharePoint also provides data analysis tools, like PerformancePoint Services and Visio Services, that enable business intelligence, and powerful new features in Excel Services.