个人做的摘记了，懒得翻译，大家自己看吧。
Razor （一个集成的新的View Engine，使代码简洁了不少。）
JavaScript Improvements
Improved Validation
Output Caching
除了支持在url 或 action-method级别的支持外，新增对页面部分(partial page)的output支持（类似于以前的
asp:Substitution控制的功能）
Previous releases of ASP.NET MVC supported output caching content at a URL or action-method level
With ASP.NET MVC V3 we are also enabling support for partial page output caching
Better Dependency Injection
You can instead just register a Dependency Injection framework with ASP.NET MVC 3
之前是需要自己去定一个类来做这个DI工作，现在只需要简单注册下类就好了。
以下是更多的新功能：
ASP.NET MVC 3 includes dozens of other nice improvements that help to both reduce the amount of code you write, and make the code you do write cleaner. Here are just a few examples:
- Improved New Project dialog that makes it easy to start new ASP.NET MVC 3 projects from templates.
- Improved Add->View Scaffolding support that enables the generation of even cleaner view templates.
- New ViewBag property that uses .NET 4’s dynamic support to make it easy to pass late-bound data from Controllers to Views.
- Global Filters support that allows specifying cross-cutting filter attributes (like [HandleError]) across all Controllers within an app.
- New [AllowHtml] attribute that allows for more granular request validation when binding form posted data to models.
- Sessionless controller support that allows fine grained control over whether SessionState is enabled on a Controller.
- New ActionResult types like HttpNotFoundResult and RedirectPermanent for common HTTP scenarios.
- New Html.Raw() helper to indicate that output should not be HTML encoded.
- New Crypto helpers for salting and hashing passwords.
- And much, much more…
原文来自：http://weblogs.asp.net/scottgu/archive/2011/01/13/announcing-release-of-asp-net-mvc-3-iis-express-sql-ce-4-web-farm-framework-orchard-webmatrix.aspx#7692548
