Here's a few additional features in ASP.NET MVC 3
- Razor view engine (Widely seen as a vast improvement on the previous ASPX view engine)
- Improved model validation with unobtrusive JavaScript and jQuery support. Including new remote validation amoungst others
- Partial page output caching.
- Dependency Injection Improvements, new IDependencyResolver`
More imporvements and details can be found at ASP.NET MVC site along with Scott Guthrie's anouncment
But only you can evaluate whether you should upgrade or not for your specific project.
It should be noted (as Robert Koritnik commented) that MVC 3 requires .NET 4.