To prevent cross-site scripting, it's important to encode data before outputting it.
Up until now, it has been quite hard to ensure you're encoding everywhere throughout your app.
It's great to see the new syntax in ASP.NET 4 to automatically encode:
First Name: <%: Model.FirstName %>
Last Name: <%: Model.FirstName %>
<%: Html.TextBox("FirstName") %>
<%: Html.TextBox("LastName") %>
(From Phil Haack's blog)
This means that for all new web applications, you can build using <%: %> instead of <%= %>, which is great for ASP.NET MVC applications where that syntax is common.
For older applications you will be able to opt in to the new encoding syntax, but your old code will keep working exactly as it already does (perhaps insecurely, if you're not encoding!)
Here's hoping that we'll be able to replace the standard HtmlEncode with the AntiXSS goodness I described here: