Migrating to Asp.Net 4.0???

Have you been working on asp.net 2.0 or 3.5 and now thinking about migrating to Asp.Net 4.0? If yes then let me tell me that there are breaking changes from asp.net 3.5 to asp.net 4.0.. I call them breaking changes since the changes are lot and were very much needed.. To know more about what's new

http://www.asp.net/learn/whitepapers/aspnet4

And yes there are breaking changes as well i.e. changes that will actually break your existing website and for which you actually need to change youe existing implentation.

For a complete list refer

http://www.asp.net/learn/whitepapers/aspnet4/breaking-changes