That's seems strange to me that the Dictionary.Item() method throws KeyNotFoundException while IDictionary.Item() does not. And Dictionary implements this interface. So why there is such implemenation that I image breaks interface convention?
(Also the symbol type structure which I image the idea behind Dictionary seems to have the convention that the method should not throw the exception.)
IDictionary is not generic, for example a HashTable implements it. So a HashTable's key and it's value are objects, hence it can (and indeed does) return null if a key was not found. A IDictioanary<TKey, TValue>'s value can be a value type, hence it has no "error"-value like null which can be returned instead.