Differences between .NET 4.0 and .NET 4.5 in High level in .NET

What is new in .NET Framework 4.5 & What's new and expected in .NET Framework 4.5:

• Support for Windows Runtime
• Support for Metro Style Applications
• Support for Async Programming
• Garbage Collector Improvements
• Faster ASP.NET Startup
• Better Data Access Support
• WebSockets Support
• Workflow Support - BCL Support

differences in ASP.NET in these frameworks

Compare What's New in ASP.NET 4 and Visual Web Developer and What's New in ASP.NET 4.5 and Visual Studio 11 Beta:

Asp.net 4.0

• Web.config File Refactoring
• Extensible Output Caching
• Auto-Start Web Applications
• Permanently Redirecting a Page
• Shrinking Session State
• Expanding the Range of Allowable URLs
• Extensible Request Validation
• Object Caching and Object Caching Extensibility
• Extensible HTML, URL, and HTTP Header Encoding
• Performance Monitoring for Individual Applications in a Single Worker Process
• Multi-Targeting
• etc

And for Asp.net 4.5 there is also a long list of improvements:

• Asynchronously Reading and Writing HTTP Requests and Responses
• Improvements to HttpRequest handling
• Asynchronously flushing a response
• Support for await and Task-Based Asynchronous Modules and Handlers

differences in C# also in these frameworks

Go Through C# 4.0 - New C# Features in the .NET Framework and What's New for Visual C# in Visual Studio 11 Beta.

Edit:
The languages documentation for C# and VB breaking changes:

VB: Visual Basic Breaking Changes in Visual Studio 2012

C#: Visual C# Breaking Changes in Visual Studio 2012

Hope this help you get what are you looking for..

.NET Framework 4

Microsoft announced the intention to ship .NET Framework 4 on 29 September 2008. The Public Beta was released on 20 May 2009.

• Parallel Extensions to improve support for parallel computing, which target multi-core or distributed systems. To this end, technologies like PLINQ (Parallel LINQ), a parallel implementation of the LINQ engine, and Task Parallel Library, which exposes parallel constructs via method calls., are included.
• New Visual Basic .NET and C# language features, such as implicit line continuations, dynamic dispatch, named parameters, and optional parameters.
• Support for Code Contracts.
• Inclusion of new types to work with arbitrary-precision arithmetic (System.Numerics.BigInteger) and complex numbers (System.Numerics.Complex).
• Introduce Common Language Runtime (CLR) 4.0.

After the release of the .NET Framework 4, Microsoft released a set of enhancements, named Windows Server AppFabric, for application server capabilities in the form of AppFabric Hosting and in-memory distributed caching support.

.NET Framework 4.5

.NET Framework 4.5 was released on 15 August 2012., a set of new or improved features were added into this version. The .NET Framework 4.5 is only supported on Windows Vista or later. The .NET Framework 4.5 uses Common Language Runtime 4.0, with some additional runtime features.

1. .NET for Metro style apps

Metro-style apps are designed for specific form factors and leverage the power of the Windows operating system. A subset of the .NET Framework is available for building Metro style apps for Windows 8 using C# or Visual Basic. This subset is called .NET APIs for apps. The version of .NET Framework, runtime and libraries, used for Metro style apps is a part of the new Windows Runtime, which is the new platform and application model for Metro style apps. It is an ecosystem that houses many platforms and languages, including .NET Framework, C++ and HTML5/JavaScript.

2. Core Features

• Ability to limit how long the regular expression engine will attempt to resolve a regular expression before it times out.
• Ability to define the culture for an application domain.
• Console support for Unicode (UTF-16) encoding.
• Support for versioning of cultural string ordering and comparison data.
• Better performance when retrieving resources.
• Zip compression improvements to reduce the size of a compressed file.
• Ability to customize a reflection context to override default reflection behavior through the CustomReflectionContext class.

3. Managed Extensibility Framework (MEF)

• Support for generic types.
• Convention-based programming model that enables you to create parts based on naming conventions rather than attributes.
• Multiple scopes.

4. Asynchronous operations

In the .NET Framework 4.5, new asynchronous features were added to the C# and Visual Basic languages. These features add a task-based model for performing asynchronous operations.

5. ASP.NET

• Support for new HTML5 form types.
• Support for model binders in Web Forms. These let you bind data controls directly to data-access methods, and automatically convert user input to and from .NET Framework data types.
• Support for unobtrusive JavaScript in client-side validation scripts.
• Improved handling of client script through bundling and minification for improved page performance.
• Integrated encoding routines from the AntiXSS library (previously an external library) to protect from cross-site scripting attacks.
• Support for WebSocket protocol.
• Support for reading and writing HTTP requests and responses asynchronously.
• Support for asynchronous modules and handlers.
• Support for content distribution network (CDN) fallback in the ScriptManager control.

6. Networking

• Provides a new programming interface for HTTP applications: System.Net.Http namespace and System.Net.Http.Headers namespaces are added.
• Other improvements: Improved internationalization and IPv6 support. RFC-compliant URI support. Support for Internationalized Domain Name (IDN) parsing. Support for Email Address Internationalization (EAI).

7. Windows Presentation Foundation (WPF)

• The new Ribbon control, which enables you to implement a ribbon user interface that hosts a Quick Access Toolbar, Application Menu, and tabs.
• The new INotifyDataErrorInfo interface, which supports synchronous and asynchronous data validation.
• New features for the VirtualizingPanel and Dispatcher classes.
• Improved performance when displaying large sets of grouped data, and by accessing collections on non-UI threads.
• Data binding to static properties, data binding to custom types that implement the ICustomTypeProvider interface and retrieval of data binding information from a binding expression.
• Repositioning of data as the values change (live shaping).
• Better integration between WPF and Win32 user interface components.
• Ability to check whether the data context for an item container is disconnected.
• Ability to set the amount of time that should elapse between property changes and data source updates.
• Improved support for implementing weak event patterns. Also, events can now accept markup extensions.

8. Windows Communication Foundation (WCF)

In the .NET Framework 4.5, the following features have been added to make it simpler to write and maintain Windows Communication Foundation (WCF) applications:

• Simplification of generated configuration files.
• Support for contract-first development.
• Ability to configure ASP.NET compatibility mode more easily.
• Changes in default transport property values to reduce the likelihood that you will have to set them.
• Updates to the XmlDictionaryReaderQuotas class to reduce the likelihood that you will have to manually configure quotas for XML dictionary readers.
• Validation of WCF configuration files by Visual Studio as part of the build process, so you can detect configuration errors before you run your application.
• New asynchronous streaming support.
• New HTTPS protocol mapping to make it easier to expose an endpoint over HTTPS with Internet Information Services (IIS).
• Ability to generate metadata in a single WSDL document by appending ?singleWSDL to the service URL.
• Websockets support to enable true bidirectional communication over ports 80 and 443 with performance characteristics similar to the TCP transport.
• Support for configuring services in code.
• XML Editor tooltips.
• ChannelFactory caching support.
• Binary encoder compression support.
• Support for a UDP transport that enables developers to write services that use "fire and forget" messaging. A client sends a message to a service and expects no response from the service.
• Ability to support multiple authentication modes on a single WCF endpoint when using the HTTP transport and transport security.
• Support for WCF services that use internationalized domain names (IDNs).

9. Tools

• Resource File Generator (Resgen.exe) enables you to create a .resw file for use in Windows Store apps from a .resources file embedded in a .NET Framework assembly.
• Managed Profile Guided Optimization (Mpgo.exe) enables you to improve application startup time, memory utilization (working set size), and throughput by optimizing native image assemblies. The command-line tool generates profile data for native image application assemblies.

For more information and access to reference links, please visit:

• .NET Framework version history
• What's New in the .NET Framework 4.5
• .NET Framework Versions and Dependencies

===========.Net 4.5 Poster=========

You can find the latest features of the .NET Framework 4.5 beta here

It breaks down the changes to the framework in the following categories:

• .NET for Metro style Apps
• Portable Class Libraries
• Core New Features and Improvements
• Parallel Computing
• Web
• Networking
• Windows Presentation Foundation (WPF)
• Windows Communication Foundation (WCF)
• Windows Workflow Foundation (WF)

You sound like you are more interested in the Web section as this shows the changes to ASP.NET 4.5. The rest of the changes can be found under the other headings.

You can also see some of the features that were new when the .NET Framework 4.0 was shipped here.

This answer may be late reply but it will be useful for seeing this topic in future.

The features of .NET framework 4.5 can be seen in the following link.

To summarize:

• Installation

  .NET Framework 4.5 does not support Windows XP or Windows Server 2003, and therefore, if you have to create applications that target these operating systems, you will need to stay with .NET Framework 4.0. In contrast, Windows 8 and Windows Server 2012 in all of their editions include .NET Framework 4.5.

• Support for Arrays Larger than 2 GB on 64-bit Platforms
• Enhanced Background Server Garbage Collection
• Support for Timeouts in Regular Expression Evaluations
• Support for Unicode 6.0.0 in Culture-Sensitive Sorting and Casing Rules on Windows 8
• Simple Default Culture Definition for an Application Domain
• Internationalized Domain Names in Windows 8 Apps