Why should one upgrade to .Net 4.0
1. Named Arguments and Optional Parameter in C# 4.0
I have used this feature in old VB versions and loved it. I really missed it in C#.
Should make code more readable
2. URL Routing with ASP.Net 4 Web Forms.
3. Built in Chart Controls (though I love ChartFx controls)
5. Pinned DataTips
6. VS2010 Debugger Improvements (Breakpoints Labeling, Data Tips and Import/Export)
Data Tip looks like a very cool feature . It should make debugging more easy.
You can add the notes. I liked the noted added by Scott to demonstrate the capabilities.
7. Clean Web.Config
Do you hate your Web.Config? I hate mine.
Important Note: .Net 4.0 includes a new version of the CLR and a new .NET 4 specific machine.config(which is installed side-by-side with the one user by .NET 2, .NET 3.0 and .NET 3.5)
8. Cleaner HTML Markup with ASP.Net 4.0 Web Forms – Client IDs
Good bye to ctl00_ContentPlaceholder1_ListView1_ctrl0_Label1 ids