In the past selecting a web framework for .NET languages was a non-issue. Your choice was between pure ASP.NET or a hybrid design that mixed classic ASP with ASP.NET. And even that was seen as a temporary hack rather than a conscious choice. But once that was done, you have to choose between C# or VB.
Look at any other platform and the situation is practically reversed. You start with a language, be that Java, Ruby, Python, Smalltalk, etc., and each has easily a dozen language-specific frameworks. By settling on a non-.NET platform, you have given up the ability to leverage multiple languages and gained the ability to select the right web framework for you.
Now so far I have been ignoring the fringe stuff like MonoRail. I do this because most .NET developers ignore them as well. There simply are not enough people using these frameworks to justify vendor or open source support. It is hard to overstate the huge library of controls and templates available to ASP.NET developers. No other .NET web framework comes anywhere near.
In fact, one could probably get away with saying that no other web framework for any platform comes close. Some, like Ruby on Rails, were just beginning to introduce the concept of controls or "widgets" in 2006.
But things are changing in the Microsoft world. Ruby on Rails, MonoRail and other MVC projects like it haven't been completely ignored. The cries of jealous .NET developers have been heard and Microsoft is developing a MVC framework for ASP.NET that completely discards the old WebForms model.
With this comes a hard choice. Do you give up your control libraries, both the ones you developed internally and those you bought, in order to gain the advantages of MVC? Is getting rid of the twin evils of view state and naming containers worth starting over on everything else?