Possible Duplicate:
Breaking changes in .NET 4.0
What are hidden problems or possible bugs when migrating C# project from .NET 3.5 to .NET 4.0?
I have one example on which I lost few hours of debugging to find real cause:
String.Trim() removed Utf8 BOM on 3.5, but this is changed in 4.0, so my program malfunctioned because of some library which was not handling UTF8 files on correct way and BOM ended as part of strings.
http://msdn.microsoft.com/en-us/library/t97s7bs3.aspx
I am looking for similar changes before they come out as bugs.
This document has a list of .NET 4 changes: http://msdn.microsoft.com/en-us/library/ee941656.aspx .
Also see previous SO question: Breaking changes in .NET 4.0