I started to use ASP.NET MVC, and I have to say, it's incredibly easy to learn and use. Development time is cut with at least 30-40% and I can spend more time on implementing the business logic and data access.Routing is another thing that have been improved, instead of using pure query strings, you build routes. This enables you to build clean URLs in a very simple way. As a ex. you would write a link as /Blog/Edit/4232 instead of /Blog/Edit.aspx?id=4232.
You had the capability to do routing before MVC, but MVC simplifies it alot.
I don't know why you aren't able to use the ASP.NET Web forms built in theming when using MVC, you have to build your own solution to enable theming. Well, the probable explanation most likely lies within that MVC embraces the HTTP stateless mode instead of trying to hide it within layers of abstractions. Fortunate, there's a lot custom solutions out on the net that solves the problem.
Some great resources to get started with ASP.NET MVC
ASP.NET MVC - Main page
Scott Gu's Weblog