If you migrate from Exchange 2010 to Exchange 2013, you lose the Open as Web Page option for viewing Office and PDF documents in Webmail on computers that didn't have Office installed. That means users accessing email via Outlook Web Access (OWA), aka Webmail, in a browser won't be able to see previews of Word, Excel, PowerPoint, OneNote, and PDF documents as they had been able to.
But Microsoft didn't really remove this capability, known as WebReady Document Viewing -- it just moved it to a new server product, known as Office Web Apps Server (OWAS). In Exchange 2013, SharePoint 2013, and Lync 2013, the server makes a Web App Open Platform Interface (WOPI) call to the OWAS server, and the OWAS server renders the documents.
Unlike Exchange 2010's WebReady Document Viewing, OWAS's WebReady Document Viewing also works with SharePoint and Lync. For SharePoint only, it also lets users edit their Office documents in the browser. And the document fidelity in OWAS is much better than it was in Exchange 2010's OWA.
But setting up OWAS takes more work than required for WebReady Document Viewing in Exchange 2010.
The actual installation of Office Web Apps Sever is a piece of cake. You can install it in Windows Server 2008 R2 or Windows Server 2012. Follow the instructions posted at Microsoft's TechNet site on how to deploy Office Web Apps Server to ensure you're properly configured, and run the PowerShell cmdlet noted there. Once the server is ready for OWAS, download the installer image file and run it -- it's an easy Next, Next, Finished kind of install.