I ran into a scenario this week where I needed to build my own expression to query a database and thought that it could make for an interesting post - maybe we could all learn something about LINQ and expressions together.
IEnumerable and collection initializers. You probably use them every day (if you write C# code at least) without thinking about the implementation details too much. And honestly - neither did I. But after I got a null reference exception (by accident, of course) whilst doing an assignment to a collection I was reminded about some of the interesting parts of IEnumerable - and some of the fun stuff you can do with it.