What's new in .net 4.0?

1. Support for multiple CLR versions running in the same process
2. Simplified security model
3. Improved GC
4. Improved profiling - Able to attach performance/memory profilers.
5. Improved debugging support
6. Support for Windows Error Reporting mini-dumps
7. Code Contracts (http://research.microsoft.com/en-us/projects/contracts/)
8. Parallel Extensions - (http://blogs.msdn.com/pfxteam/archive/2008/10/31/9026988.aspx)
9. BigInteger
10. Memory Mapped IO Files
11. Improved ResourceManager
12. More support for user’s UI settings (preferred language, not just current UI culture)
13. Improved Compression algorithm
14. No 4 GB limit now.
15. Improved String class - (Culture-agnostic methods by default)
16. Variance Annotations - http://channel9.msdn.com/pdc2008/TL16/
17. Tubles - For better language interop
18. SortedSet - Addition to ISet
19. Improved File System Enumeration - (System.IO.Directory wont return array and instead returns IEnumerable collection.
20. Improved Microsoft.Win32.RegistryKey support for 64 bit windows.
21. Improved Security - http://msdn.microsoft.com/en-us/library/dd233103(VS.100).aspx
22. Access to Event Tracing for Windows (http://msdn.microsoft.com/en-us/library/dd264810(VS.100).aspx, http://msdn.microsoft.com/en-us/library/dd264809(VS.100).aspx)
23. Lazy Initialization - System.Lazy
24. Dynamic Language Runtime - http://msdn.microsoft.com/en-us/library/dd233052(VS.100).aspx
25. In-Process side by side execute - http://msdn.microsoft.com/en-us/library/dd233120(VS.100).aspx
26. Extensible output caching in ASP.net
27. Auto Start Web Applications
28. Permanently redirecting a page
29. Shrinking session state
30. View state for individual controls
31. Entity Templates
32. Enhanced Ajax Support