Note: this entry has moved.
A couple months ago I posted about a major regression happening in ASP.NET v2. It's amazing that even after an overwhelming difference of 10 to 1 people voting to keep the Components tray in the designer, which allowed wizards, designer verbs, and extender providers to provide an incredible powerful enhanced visual experience in the IDE, Microsoft decided not to fix it.
So, you can kiss bye-bye to your UI-technology agnostic components and IDE integration. You'll have to keep two codebases and use the ugly gray-rendered "controls" (can't understand why you have to inherit from Control or WebControl if you're not doing any UI stuff, and why it has to live in the HTML design surface). A huge step backwards in what seemed to me like an important evolutionary step towards software factories based on highly reusable components.
They definitely ruined my day. But I plan to publish an extensive article of what you'll lose, and all the features you can exploit from this powerful model. Hopefully, if I get enough developers to use it, they will put more effort in bringing it back.