New features

Server-side event handlers and ctx.ClientSide can take any fixed F# quotation now and translates it to JavaScript.
New implementation for WebSharper.Sitelets.Router type, fully composable and also working on the client-side. WebSharper.UI adds helpers to use it for client-side routing. This allows generating safe links on the client for server-side endpoints, also on the server to states handled by the client.

See the details of Sitelet and Router features in the C# and F# documentation.

WebSharper.UI

WebSharper.UI.Next has been renamed to WebSharper.UI, affecting both package and namespace naming. For C# projects, an additional package reference for WebSharper.UI.CSharp is needed, and .CSharp has been removed from all namespace names, so now no extra usings are needed to get the C#-oriented extension methods. A compatibility package WebSharper.UI.Next 4.1 still exists for immediate back-compatibility, switching to WebSharper.UI is recommended for new features and fixes.
For composing HTML in F#, the ...Attr family of functions have been renamed to match just the name of the tag, while the previous functions taking no Attr list have been removed. So previous div [text "Hello"] becomes div [] [text "Hello"] with WebSharper.UI.
An nice and easy syntax is available for creating reactive values. If x and y are both Var<int> or View<int> then V(x.V + x.Y) is eqivalent to using View.Map2 (+). Also within F# text or Attr. functions and any C# HTML combinators, the logic for V(...) is applied automatically, so no extra wrapping is needed.
Template types (created either by the C# code generator or the F# type provider) have a new .Vars property, exposing all the variables that has been bound to inputs in the template. If you specify no binding, a Var will be implicitly created for the input.
The IRef type has been removed and Var is now an abstract class.

See the details of UI features in the C# and F# documentation.

Fix using <UseDownloadedResources>True</UseDownloadedResources> in web.config, now local script links are correct for any sub-page.
Expanded WebSharper.Testing to expose more equality test alternatives provided by QUnit.
Better handling of F# trait calls (using a member constraint inside an inline function). Now an overloaded method is resolved based on signature.
Optimization of remoting protocol: array of types are not included when it is empty.

Attributes previously in WebSharper.Sitelets namespace and assembly are now in WebSharper namespace and in WebSharper.Core.dll.
A Sitelet's Router field is now of type IRouter, compatible in capabilities with the old Router record. The new Router type offers more combinators and is implementing IRouter.