There probably is a heavy temptation for managed services providers who host large data centers that are sold as Clouds, to continue to develop Full Trust Web Parts even when going to SharePoint 2013. Web Parts are what coders are used to, they may have a large library of existing Web Parts as WSP files that run in full trust. This is possible, and it means working pretty much as you always did.
But if you are like most managed service providers I know you have some kind of internal structure between your infrastructure providers who build and maintain the farms and the application and solutions providers who face the customers and develop SharePoint from a raw out of the box in to something more specific and rich for their needs. By adopting a SharePoint 2013 App Model you can move away from this. Two teams, even if they work in the same company can concentrate on what they are best at. Infrastructure can concentrate on meeting SLAs, providing backup and DR and applying patches.
Solution providers can create partial trust solution that use .NET SCOM or REST and JSON to create rich interfaces that are hosted in other domains, perhaps even other server farms with different domain controllers. You now no longer have the tight link between the people installing SharePoint farm and the developers of solution, in fact you can separate your teams so that one applications team can create solutions, potentially in PHP or Ruby, that work off more than one SharePoint 2013 farm.
So it makes solid business sense even for established big players to embrace JQuery and RESTful and build App that run in separate domains using OAuth and OData to communicate with SharePoint, creating UI in HTML 5 and CSS 3
The potential is perhaps even greater for smaller firms. Now a smaller firm with a team of talented web developers who know about JQuery, mobile, tablets, RESTful, OData and rapid design can muscle in on a space that previously only the provider could have. And I speak from hard earned experience when I say that the big guys will be more than happy to have third party code isolated in its own IIS server rather than installed on perm SharePoint farm.
- First class Sharepoint developer tools
- Faster solutions, debuggin and tuning
- Choice of development workstation
Rich Platform Services
- Comprehensive data access
- Line of business integration
- Workflow advances
- Application lifecycle management
- Standardized solution packaging
- Deploy with agility and stability