I predict that the C# 4 “dynamic” keyword will be the most abused feature.
It will be used for everything from ducktyped dependency injection, dynamic dictionaries and about a million Rails like frameworks.
All in a very non refactor friendly way.
…It will rarely be used to interop with dynamic .NET languages.
The least used will be the co/contra variance feature.
In 2011 not even the C# team will remember it exists.
The most frustrating feature will be code contracts.
Developers will cry out in pain when they realize that it might not be so easy as they expected it to be.
I’m just guessing :-)
Roger Johansson Blog 
I’m personally very happy that co/contra variance finally comes to C# (and VB.NET ?) (well, although most of my code is running on .NET 2.0 so …).
I think people will use it, they will just not notice it.
I’m also a Java programmer and I did use it more than a few times …
I cannot agree more on the “dynamic” keyword. It’s just smacks way to much of “variant” in VB6. I think it’s probably the worse feature to introduce into .Net (not that my opinion counts for anything :-) )