Architecture Change in SP2013 or improvements in 2013

1. Few improvements like 

• Shredded Storage - Refer Here
• In SP 2010: 
• When file uploaded in Sgarepoint only the bits that have chnaged is send from client to WFE. because of incremental update not there in SQL for SP 2010 it will do like 
• pull the entire file from WFE
• Merge the changes back into it
• write entire file back to SQL
• In SP 2013: 
• File is broken in pieces in & stored in SQL
• while update only the updated bits are touched
• No more round trip of file to WFE and back
• SQL improvemnets
• reduced data redundancy using advanced indexing feature of SQL 2008 R2
• support of wide list, i.e single item span multiple rows in the DB to hold data.
• Reduce scenario to invoke full table scan.
• Request Management - Refer Here
• This feature gives SharePoint knowledge of and also more control on incoming requests.
• Nature of incoming requests like user agent, requested URL, Source IP.
• goals of request management are as below:
• RM can route request to WFE with better health.
• can identify harmful requests and can deny immediately
• Can prioritize requests 
• Can send all requests of specific type to specific machine, like search type request.
• Isolated traffic can help troubleshoot errors on one machine
• Can send heavy requests to more powerful WFEs.
• 
• Cache service - Refer Here
• DCS based on windows server AppFabric Distributed Caching.
• SP 2013 supports the version of caching that it ships, you can't independently upgrade it.
• Config DB keeps track of which machines in the farm are running the cache service. 
• Distributed Cache service is installed on each server in the farm when SharePoint is installed
• Sharing
• Themes

2. Service application level changes 

• Office web app is no longer a service application
• It is a separate application and recommended to be installed as separate farm
• Web analytic is also not a service, its part of search now.
• Few new services introduced  
• App Management Service
• Allow to install SharePoint Apps from office marketplace or App catalog
• SharePoint Translation Service
• Does language translation of word, PPT, XLIFF files to HTML
• Work Management Service
• Task aggregation across system like SharePoint, Exchange, Project
• Azure Workflow Server
• Its not exactly a service application
• Provides an externalized host using REST and OAuth to run workflows.

3. Enterprise content Management

• Site –level retention policies
• Discover Center
• eDiscover capabilities
• Team folders to integrate with exchange

4. Web Content Management

• Support the tools and workflow designers use
• Variations & Content Translation
• Search Engine Optimization
• Cross Site Publishing
• Video & Embedding
• Image renditions
• Clean Urls
• Metadata navigation

5. Social

• Micro blogging – share and follow
• Activity Feeds – view activities related to content
• Communities
• Discussions
• Blogs

7. Search

• Personalized search results based on search history
• Rich contextual reviews

8. Business Intelligence Enhancements
9. Mobile
10. Remove API Enhancements
11. Other Considerations 

• Stretched farms are no longer supported in SharePoint 2013. (Stretched means different data centers with less than 1ms latency)
• All servers in the farm must be in the same data center now. (For 100% fidelity in 100% of features)