I would start with saying CRM 2013 has come with many UI
features to make it much more user friendly than the Previous versions.
To mention a few, 
·        
Bing Maps
embedded control
·        
The Flat
User Interface
·        
The Quick
Create feature, which makes creation of Task, Phone call, Email, Account,
Contact etc.  easy , While you are in the
middle of editing a Case, If you need to create a Phone call Activity, You can
quickly do that!
·        
The Quick
View Feature- Users can see the Parent entity details in the Child record

·        
Inline
Notifications -On the Form, the errors/warnings/information can be seen in
line with the field

·        
Simplified
Navigation, improving the Form Body view
·        
The
Auto-Save feature, where-in, a Record is automatically saved by CRM every
30 Seconds(The form viewed by the User), also on exit  from the form
·        
Business
Process Flow, which can be a visual indicative of a Flow

·        
Social
Pane,Yammer and Lync Integration,  One
can follow a User or a record in the same organization
There are other exciting features for the developers like:
·        
Portable
Business Logic\Business Rules Basic Validations/ other Jscript
functionalities like Field Enabling Disabling, Hiding/Showing , Populating can
be done without necessity to write any code, this makes it easy for CRM
functional analysts to work on it without any Developer’s help
·        
Real Time
Workflows- These are Synchronous Workflows, Using this, developers
configure a Workflow instead of writing Custom Code Plugin wherever System Out
Of the Box feature suffices 
·        
Actions Using
these, Custom Messages can be
created, hence frequently used operations can be packed into actions and can be
called from Plugins/Custom Workflows.
These can be defined for a Particular entity/
As Global



