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ASP.NET has great caching and load balancing features that make it suitable for high load Web sites.
It's also a very flexible, well-designed framework. If you don't like part of it, or think its performance intensive for your high load application, you can just throw out that layer and replace it with your own. If you don't like WebControls and Pages due to its overhead, you are able to write your own
Basically, I think ASP.NET MVC is going to be a better platform for high load and customized Web sites, since it can be more hand tuned (relative to ASP.NET pages that try to abstract away the resulting HTML, for good reasons) and works better with AJAX applications. WebControls and ViewState are two parts that usually face criticism that are pretty much gone away in MVC.